add gap answer

docs/manual.typ

@@ -393,3 +393,18 @@ Which of these answers are even?
     style: tidy.styles.minimal,
   )
 }
+
+== `scrutinize.task-kinds.gap`
+
+#{
+  let module = tidy.parse-module(
+    read("/src/task-kinds/gap.typ"),
+    label-prefix: "gap.",
+    scope: scope,
+  )
+  tidy.show-module(
+    module,
+    sort-functions: none,
+    style: tidy.styles.minimal,
+  )
+}

src/task-kinds/gap.typ

@@ -0,0 +1,41 @@
+#let _grid = grid
+
+/// An answer filled in a gap in a text. If the document is not in solution mode, the answer is
+/// hidden but the height of the element is preserved.
+///
+/// Example:
+///
+/// #task-example(lines: "2-", ```typ
+/// #import task-kinds.gap: gap
+/// #set par(leading: 1em)
+/// This is a #gap(width: 2cm)[difficult] question \
+/// and it has #gap(width: 1.2cm, stroke: "box")[two] lines.
+/// ```)
+///
+/// - answer (content): the answer to (maybe) display
+/// - width (auto, relative): the width of the region where an answer can be written
+/// - stroke (none, string, stroke): the stroke with which to mark the answer area. The special
+///   values `"underline"` or `"box"` may be given to draw one or four border lines with a default
+///   stroke.
+/// -> content
+#let gap(answer, width: auto, stroke: "underline") = context {
+  import "../solution.typ"
+
+  let stroke = stroke
+  assert(
+    type(stroke) != str or stroke in ("underline", "box"),
+    message: "for string values, only \"underline\" or \"box\" are allowed",
+  )
+  if stroke == "underline" {
+    stroke = (bottom: 0.5pt)
+  } else if stroke == "box" {
+    stroke = 0.5pt
+  }
+
+  let answer = answer
+  if (not solution.get()) {
+    answer = hide(answer)
+  }
+
+  box(width: width, stroke: stroke, outset: (y: 0.4em), align(center, answer))
+}

src/task-kinds/mod.typ

@@ -1,2 +1,3 @@
 #import "choice.typ"
 #import "free-form.typ"
+#import "gap.typ"