1 files changed, 56 insertions, 1 deletions

diff --git a/gui/internal/model/forms.go b/gui/internal/model/forms.go
index f0a3983..290bbc8 100644
--- a/gui/internal/model/forms.go
+++ b/gui/internal/model/forms.go
@@ -88,16 +88,46 @@ func (r *Rules) ReplaceForm(i int, text string) error {
 // DeleteForm removes form i and the comment lines directly above it - its
 // block (GUI design §5.1). A comment separated from the form by a blank
 // line belongs to the file, not to the form, and stays.
+//
+// A block is whole lines, which is wrong when another form shares one:
+// deleting the first of "(exclude A) (exclude B)" would take B as well,
+// silently, leaving a file that still parses and still checks clean. When
+// the block would reach another form, only the form's own text goes, with
+// the spaces that followed it.
 func (r *Rules) DeleteForm(i int) error {
-	f, err := r.form(i)
+	forms, err := r.Forms()
 	if err != nil {
 		return err
 	}
+	if i < 0 || i >= len(forms) {
+		return fmt.Errorf("model: no form %d", i)
+	}
+	f := forms[i]
 	start, end := r.block(f)
+	if r.blockTouchesAnotherForm(forms, i, start, end) {
+		start, end = f.Pos.Offset, f.End.Offset
+		for end < len(r.Text) && (r.Text[end] == ' ' || r.Text[end] == '\t') {
+			end++
+		}
+	}
 	r.Text = join(r.Text[:start], r.Text[end:])
 	return nil
 }
 
+// blockTouchesAnotherForm reports whether the span start..end covers any
+// part of a form other than forms[i].
+func (r *Rules) blockTouchesAnotherForm(forms []Form, i, start, end int) bool {
+	for j, other := range forms {
+		if j == i {
+			continue
+		}
+		if other.Pos.Offset < end && other.End.Offset > start {
+			return true
+		}
+	}
+	return false
+}
+
 // MoveForm moves form i one place up (delta -1) or down (delta 1), with its
 // comments. Moving past either end does nothing.
 func (r *Rules) MoveForm(i, delta int) error {
@@ -222,6 +252,31 @@ func (r *Rules) block(f Form) (start, end int) {
 	return start, end
 }
 
+// clearForm removes the form spanning start..end from text. It takes the
+// whole line - with the comment trailing it, and the blank it would leave
+// behind - only when the line holds nothing else. A setting written beside
+// something else, which is what the Settings window itself writes when it
+// creates "(defaults\n  (case ignore))", loses only itself: taking the line
+// took the closing paren of defaults with it and broke the file.
+func clearForm(text string, start, end int) string {
+	ls, le := lineStart(text, start), lineEnd(text, end)
+	before := strings.TrimSpace(text[ls:start])
+	after := text[end:le]
+	if i := strings.IndexByte(after, ';'); i >= 0 {
+		// A comment after it on the same line describes it, and goes too.
+		after = after[:i]
+	}
+	if before == "" && strings.TrimSpace(after) == "" {
+		return join(text[:ls], text[le:])
+	}
+	// Something else shares the line: drop the form and the spaces that
+	// followed it, and nothing more.
+	for end < len(text) && (text[end] == ' ' || text[end] == '\t') {
+		end++
+	}
+	return text[:start] + text[end:]
+}
+
 // lineStart is the offset just after the newline before at.
 func lineStart(s string, at int) int {
 	if at > len(s) {