diff --git a/sourcing/edit.py b/sourcing/edit.py
new file mode 100644
index 0000000..653c458
--- /dev/null
+++ b/sourcing/edit.py
@@ -0,0 +1,89 @@
+import attr
+
+class EditOutOfRange(Exception):
+    pass
+
+def apply_delete(current_spans, edit):
+    assert edit
+    if not current_spans:
+        raise ValueError('edit is out of bounds')
+
+    spans = []
+    pos = 0
+    edit_end = edit['start'] + len(edit['old'])
+
+    cur_span = current_spans.pop(0)
+    while pos + cur_span.length < edit['start']:
+        spans.append(cur_span)
+        pos += cur_span.length
+        cur_span = current_spans.pop(0)
+
+    if edit['start'] > pos:
+        new_span = attr.evolve(cur_span, length=edit['start'] - pos)
+        spans.append(new_span)
+
+    while pos + cur_span.length < edit_end:
+        pos += cur_span.length
+        cur_span = current_spans.pop(0)
+
+    if pos + cur_span.length != edit_end:
+        offset = cur_span.start - pos
+        new_start = offset + (edit_end - pos)
+        diff = new_start - cur_span.start
+        new_span = attr.evolve(cur_span,
+                               length=cur_span.length - diff,
+                               start=new_start)
+        spans.append(new_span)
+
+    spans += current_spans
+    return spans
+
+def apply_insert(current_spans, edit):
+    if not current_spans and edit['0'] == 0:
+        return edit['span']
+
+    pos = 0
+    spans = []
+    cur_span = current_spans.pop(0)
+    while pos + cur_span.length < edit['start']:
+        spans.append(cur_span)
+        pos += cur_span.length
+        cur_span = current_spans.pop(0)
+
+    if edit['start'] >= pos:
+        length_a = edit['start'] - pos
+        length_b = cur_span.length - length_a
+
+        if length_a:
+            span_a = attr.evolve(cur_span, length=length_a)
+            pos += length_a
+            spans.append(span_a)
+
+        spans.append(edit['span'])
+        pos += edit['span'].length
+
+        if length_b:
+            span_b = attr.evolve(cur_span,
+                                 start=cur_span.start + length_a,
+                                 length=length_b)
+            spans.append(span_b)
+
+            pos += length_b
+    else:
+        spans.append(edit['span'])
+
+    spans += current_spans
+    return spans
+
+def apply_edits(spans, edits):
+    for edit in edits:
+        if edit['op'] == 'delete':
+            spans = apply_delete(spans, edit)
+            continue
+        if edit['op'] == 'insert':
+            spans = apply_insert(spans, edit)
+            continue
+
+    return spans
+
+
diff --git a/sourcing/span.py b/sourcing/span.py
new file mode 100644
index 0000000..08f9cd8
--- /dev/null
+++ b/sourcing/span.py
@@ -0,0 +1,18 @@
+import attr
+
+def greater_than_zero(instance, attribute, value):
+    if value <= 0:
+        raise ValueError('must be greater than 0')
+
+def is_positive(instance, attribute, value):
+    if value < 0:
+        raise ValueError('must be positive')
+
+@attr.s
+class Span:
+    url: int = attr.ib()
+    start: int = attr.ib(validator=is_positive)
+    length: int = attr.ib(validator=greater_than_zero)
+
+    def end(self) -> int:
+        return self.start + self.length
diff --git a/tests/test_edit.py b/tests/test_edit.py
new file mode 100644
index 0000000..93eb59b
--- /dev/null
+++ b/tests/test_edit.py
@@ -0,0 +1,54 @@
+from sourcing.span import Span
+from sourcing.edit import apply_edits
+import pytest
+
+def test_xanadoc_apply_delete_start():
+    spans = [Span('http://test/test', 0, 11)]
+
+    edits = [{'op': 'delete', 'start': 0, 'old': 'aaa'}]
+    spans = apply_edits(spans, edits)
+    assert spans == [Span('http://test/test', 3, 8)]
+
+def test_xanadoc_apply_delete_start_offset():
+    offset = 14
+    spans = [Span('http://test/test', offset, 11)]
+
+    edits = [{'op': 'delete', 'start': 0, 'old': 'aaa'}]
+    spans = apply_edits(spans, edits)
+    assert spans == [Span('http://test/test', offset + 3, 8)]
+
+def test_xanadoc_apply_delete_middle():
+    spans = [Span('http://test/test', 0, 11)]
+
+    edits = [{'op': 'delete', 'start': 4, 'old': 'bbb'}]
+    spans = apply_edits(spans, edits)
+    assert spans == [Span('http://test/test', 0, 4),
+                     Span('http://test/test', 7, 4)]
+
+def test_xanadoc_apply_delete_end():
+    spans = [Span('http://test/test', 0, 11)]
+
+    edits = [{'op': 'delete', 'start': 8, 'old': 'ccc'}]
+    spans = apply_edits(spans, edits)
+    assert spans == [Span('http://test/test', 0, 8)]
+
+def test_xanadoc_apply_delete_all():
+    spans = [Span('http://test/test', 0, 3)]
+
+    edits = [{'op': 'delete', 'start': 0, 'old': 'aaa'}]
+    spans = apply_edits(spans, edits)
+    assert spans == []
+
+def test_xanadoc_apply_insert_start():
+    existing_span = Span('http://test/test', 0, 8)
+    new_span = Span('http://test/new_span', 10, 4)
+    edits = [{'op': 'insert', 'start': 0, 'span': new_span}]
+    spans = apply_edits([existing_span], edits)
+    assert spans == [new_span, existing_span]
+
+def test_xanadoc_apply_insert_end():
+    existing_span = Span('http://test/test', 0, 8)
+    new_span = Span('http://test/new_span', 10, 4)
+    edits = [{'op': 'insert', 'start': 8, 'span': new_span}]
+    spans = apply_edits([existing_span], edits)
+    assert spans == [existing_span, new_span]