diff --git a/src/Makefile.am b/src/Makefile.am
index 2a10221..126f1d8 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -45,3 +45,8 @@ TESTS += parser.test
 check_PROGRAMS += parser.test
 parser_test_SOURCES = parser_test.c test.h
 parser_test_LDADD = libapfl.a
+
+TESTS += resizable.test
+check_PROGRAMS += resizable.test
+resizable_test_SOURCES = resizable_test.c test.h resizable.h
+resizable_test_LDADD = libapfl.a
diff --git a/src/resizable.c b/src/resizable.c
index 9e00733..d688978 100644
--- a/src/resizable.c
+++ b/src/resizable.c
@@ -59,15 +59,65 @@ apfl_resizable_ensure_cap_for_more_elements(size_t elem_size, void **mem, size_t
     return apfl_resizable_ensure_cap(elem_size, mem, cap, len + more_elements); // TODO: What if len + more_elements overflows?
 }
 
-bool
-apfl_resizable_append(size_t elem_size, void **mem, size_t *len, size_t *cap, const void *other_mem, size_t other_len)
-{
-    if (!apfl_resizable_ensure_cap_for_more_elements(elem_size, mem, *len, cap, other_len)) {
+bool apfl_resizable_splice(
+    size_t elem_size,
+    void **mem,
+    size_t *len,
+    size_t *cap,
+    size_t cut_start,
+    size_t cut_len,
+    const void *other_mem,
+    size_t other_len
+) {
+    if (cut_start > *len || cut_start + cut_len > *len) {
         return false;
     }
 
-    memcpy(*((char**)mem) + (elem_size * *len), other_mem, other_len * elem_size);
-    *len += other_len;
+    if (other_len > cut_len) {
+        if (!apfl_resizable_ensure_cap_for_more_elements(
+            elem_size,
+            mem,
+            *len,
+            cap,
+            other_len - cut_len
+        )) {
+            return false;
+        }
+    }
+
+    size_t src_off = cut_start + cut_len;
+    size_t dst_off = cut_start + other_len;
+
+    memmove(
+        ((char *)(*mem)) + (dst_off * elem_size),
+        ((char *)(*mem)) + (src_off * elem_size),
+        (*len - cut_start - cut_len) * elem_size
+    );
+
+    if (other_len > 0 && other_mem != NULL) {
+        memcpy(
+            ((char *)(*mem)) + cut_start * elem_size,
+            other_mem,
+            other_len * elem_size
+        );
+    }
+
+    *len += other_len - cut_len;
 
     return true;
 }
+
+bool
+apfl_resizable_append(size_t elem_size, void **mem, size_t *len, size_t *cap, const void *other_mem, size_t other_len)
+{
+    return apfl_resizable_splice(
+        elem_size,
+        mem,
+        len,
+        cap,
+        *len,
+        0,
+        other_mem,
+        other_len
+    );
+}
diff --git a/src/resizable.h b/src/resizable.h
index fe7d12c..a7c8338 100644
--- a/src/resizable.h
+++ b/src/resizable.h
@@ -18,7 +18,18 @@ bool apfl_resizable_resize(size_t elem_size, void **mem, size_t *len, size_t *ca
 bool apfl_resizable_ensure_cap(size_t elem_size, void **mem, size_t *cap, size_t want_cap);
 bool apfl_resizable_ensure_cap_for_more_elements(size_t elem_size, void **mem, size_t len, size_t *cap, size_t more_elements);
 
+bool apfl_resizable_splice(
+    size_t elem_size,
+    void **mem,
+    size_t *len,
+    size_t *cap,
+    size_t start,
+    size_t cut_len,
+    const void *other_mem,
+    size_t other_len
+);
 
 bool apfl_resizable_append(size_t elem_size, void **mem, size_t *len, size_t *cap, const void *other_mem, size_t other_len);
 
+
 #endif
diff --git a/src/resizable_test.c b/src/resizable_test.c
new file mode 100644
index 0000000..f8d6115
--- /dev/null
+++ b/src/resizable_test.c
@@ -0,0 +1,325 @@
+#include <string.h>
+
+#include "test.h"
+
+#include "resizable.h"
+
+struct splice_test_data {
+    const int *old;
+    size_t cut_start;
+    size_t cut_len;
+    const int *other;
+    const int *want;
+};
+
+static size_t
+getlen(const int *data)
+{
+    size_t len = 0;
+    for (; *data != 0; data++) {
+        len++;
+    }
+    return len;
+}
+
+static void
+run_splice_test(testctx t, struct splice_test_data st)
+{
+    size_t len = getlen(st.old);
+    int *mem = must_alloc(t, sizeof(int) * len);
+    memcpy(mem, st.old, sizeof(int) * len);
+    size_t cap = len;
+
+    size_t other_len = getlen(st.other);
+
+    if (!apfl_resizable_splice(
+        sizeof(int),
+        (void **)&mem,
+        &len,
+        &cap,
+        st.cut_start,
+        st.cut_len,
+        st.other,
+        other_len
+    )) {
+        test_fatalf(t, "splice failed!");
+    }
+
+    size_t want_len = getlen(st.want);
+
+    if (len != want_len) {
+        test_fatalf(t, "Expected len=%d, got %d", (int)want_len, (int)len);
+    }
+    if (cap < want_len) {
+        test_fatalf(t, "Expected cap>=%d, got %d", (int)want_len, (int)cap);
+    }
+
+    for (size_t i = 0; i < len; i++) {
+        int have = mem[i];
+        int want = st.want[i];
+
+        if (have != want) {
+            test_failf(t, "have(%d) != want(%d) at i=%d", have, want, (int)i);
+        }
+    }
+
+    free(mem);
+}
+
+TEST(splice_empty, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {0},
+        .cut_start = 0,
+        .cut_len = 0,
+        .other = (int[]) {0},
+        .want = (int[]) {0},
+    });
+}
+
+TEST(splice_remove_single_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 0,
+        .cut_len = 1,
+        .other = (int[]) {0},
+        .want = (int[]) {2, 3, 0},
+    });
+}
+
+TEST(splice_remove_multiple_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 0,
+        .cut_len = 2,
+        .other = (int[]) {0},
+        .want = (int[]) {3, 0},
+    });
+}
+
+TEST(splice_remove_single_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 2,
+        .cut_len = 1,
+        .other = (int[]) {0},
+        .want = (int[]) {1, 2, 0},
+    });
+}
+
+TEST(splice_remove_multiple_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 1,
+        .cut_len = 2,
+        .other = (int[]) {0},
+        .want = (int[]) {1, 0},
+    });
+}
+
+TEST(splice_remove_single_mid, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 1,
+        .cut_len = 1,
+        .other = (int[]) {0},
+        .want = (int[]) {1, 3, 0},
+    });
+}
+
+TEST(splice_replace_same_size_single_mid, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 1,
+        .cut_len = 1,
+        .other = (int[]) {42, 0},
+        .want = (int[]) {1, 42, 3, 0},
+    });
+}
+
+TEST(splice_replace_larger_size_single_mid, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 1,
+        .cut_len = 1,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {1, 10, 20, 30, 3, 0},
+    });
+}
+
+TEST(splice_replace_smaller_size_single_mid, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 1,
+        .cut_len = 2,
+        .other = (int[]) {10, 0},
+        .want = (int[]) {1, 10, 4, 0},
+    });
+}
+
+TEST(splice_replace_same_size_single_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 0,
+        .cut_len = 1,
+        .other = (int[]) {42, 0},
+        .want = (int[]) {42, 2, 3, 0},
+    });
+}
+
+TEST(splice_replace_larger_size_single_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 0,
+        .cut_len = 1,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {10, 20, 30, 2, 3, 0},
+    });
+}
+
+TEST(splice_replace_smaller_size_single_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 2,
+        .other = (int[]) {10, 0},
+        .want = (int[]) {10, 3, 4, 0},
+    });
+}
+
+TEST(splice_replace_same_size_single_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 2,
+        .cut_len = 1,
+        .other = (int[]) {42, 0},
+        .want = (int[]) {1, 2, 42, 0},
+    });
+}
+
+TEST(splice_replace_larger_size_single_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 0},
+        .cut_start = 2,
+        .cut_len = 1,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {1, 2, 10, 20, 30, 0},
+    });
+}
+
+TEST(splice_replace_smaller_size_single_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 2,
+        .cut_len = 2,
+        .other = (int[]) {10, 0},
+        .want = (int[]) {1, 2, 10, 0},
+    });
+}
+
+TEST(splice_remove_all, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 4,
+        .other = (int[]) {0},
+        .want = (int[]) {0},
+    });
+}
+
+TEST(splice_replace_all_same_size, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 4,
+        .other = (int[]) {10, 20, 30, 40, 0},
+        .want = (int[]) {10, 20, 30, 40, 0},
+    });
+}
+
+TEST(splice_replace_all_smaller_size, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 4,
+        .other = (int[]) {10, 0},
+        .want = (int[]) {10, 0},
+    });
+}
+
+TEST(splice_replace_all_larger_size, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 4,
+        .other = (int[]) {10, 20, 30, 40, 50, 60, 0},
+        .want = (int[]) {10, 20, 30, 40, 50, 60, 0},
+    });
+}
+
+TEST(splice_noop, t) {
+    for (size_t i = 0; i < 4; i++) {
+        run_splice_test(t, (struct splice_test_data) {
+            .old = (int[]) {1, 2, 3, 4, 0},
+            .cut_start = i,
+            .cut_len = 0,
+            .other = (int[]) {0},
+            .want = (int[]) {1, 2, 3, 4, 0},
+        });
+    }
+}
+
+TEST(splice_insert_begin, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 0,
+        .cut_len = 0,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {10, 20, 30, 1, 2, 3, 4, 0},
+    });
+}
+
+TEST(splice_insert_mid, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 1,
+        .cut_len = 0,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {1, 10, 20, 30, 2, 3, 4, 0},
+    });
+}
+
+TEST(splice_insert_end, t) {
+    run_splice_test(t, (struct splice_test_data) {
+        .old = (int[]) {1, 2, 3, 4, 0},
+        .cut_start = 4,
+        .cut_len = 0,
+        .other = (int[]) {10, 20, 30, 0},
+        .want = (int[]) {1, 2, 3, 4, 10, 20, 30, 0},
+    });
+}
+
+TESTS_BEGIN
+    ADDTEST(splice_empty),
+    ADDTEST(splice_remove_single_begin),
+    ADDTEST(splice_remove_multiple_begin),
+    ADDTEST(splice_remove_single_end),
+    ADDTEST(splice_remove_multiple_end),
+    ADDTEST(splice_remove_single_mid),
+    ADDTEST(splice_replace_same_size_single_mid),
+    ADDTEST(splice_replace_larger_size_single_mid),
+    ADDTEST(splice_replace_smaller_size_single_mid),
+    ADDTEST(splice_replace_same_size_single_begin),
+    ADDTEST(splice_replace_larger_size_single_begin),
+    ADDTEST(splice_replace_smaller_size_single_begin),
+    ADDTEST(splice_replace_same_size_single_end),
+    ADDTEST(splice_replace_larger_size_single_end),
+    ADDTEST(splice_replace_smaller_size_single_end),
+    ADDTEST(splice_remove_all),
+    ADDTEST(splice_replace_all_same_size),
+    ADDTEST(splice_replace_all_smaller_size),
+    ADDTEST(splice_replace_all_larger_size),
+    ADDTEST(splice_noop),
+    ADDTEST(splice_insert_begin),
+    ADDTEST(splice_insert_mid),
+    ADDTEST(splice_insert_end),
+TESTS_END