diff --git a/crates/wordmodel/src/lib.rs b/crates/wordmodel/src/lib.rs
index 664a72ef..59242ec4 100644
--- a/crates/wordmodel/src/lib.rs
+++ b/crates/wordmodel/src/lib.rs
@@ -40,6 +40,7 @@ pub mod math;
 pub mod math_latex;
 pub mod math_linear;
 pub mod math_symbols;
+pub mod math_tree;
 pub mod numbering;
 pub mod para;
 pub mod picfx;
diff --git a/crates/wordmodel/src/math_tree.rs b/crates/wordmodel/src/math_tree.rs
new file mode 100644
index 00000000..45d2c603
--- /dev/null
+++ b/crates/wordmodel/src/math_tree.rs
@@ -0,0 +1,104 @@
+//! Structural access to an equation tree: the arguments of a node, and every run in a tree.
+
+use crate::math::{Arg, MAX_DEPTH, MNode, MRun};
+
+/// Number of child arguments of a node.
+pub fn child_count(n: &MNode) -> usize {
+    match n {
+        MNode::Run(_) => 0,
+        MNode::Frac { .. } | MNode::Func { .. } | MNode::Lim { .. } => 2,
+        MNode::Rad { .. } => 2,
+        MNode::Script { .. } | MNode::Nary { .. } => 3,
+        MNode::Delim { elems, .. } => elems.len(),
+        MNode::EqArr { rows } => rows.len(),
+        MNode::Matrix { rows, .. } => rows.iter().map(|r| r.len()).sum(),
+        MNode::Acc { .. } | MNode::Bar { .. } | MNode::BorderBox { .. } | MNode::Boxed { .. } | MNode::GroupChr { .. } | MNode::Phant { .. } => 1,
+    }
+}
+
+/// Child argument `i` of a node (see [`child_count`] for the numbering: fraction num 0 / den 1;
+/// script base 0 / sub 1 / sup 2; radical degree 0 / base 1; n-ary sub 0 / sup 1 / operand 2;
+/// function name 0 / argument 1; limit base 0 / limit 1; matrices row by row).
+pub fn child(n: &MNode, i: usize) -> Option<&Arg> {
+    match (n, i) {
+        (MNode::Frac { num, .. }, 0) => Some(num),
+        (MNode::Frac { den, .. }, 1) => Some(den),
+        (MNode::Script { base, .. }, 0) => Some(base),
+        (MNode::Script { sub, .. }, 1) => Some(sub),
+        (MNode::Script { sup, .. }, 2) => Some(sup),
+        (MNode::Rad { deg, .. }, 0) => Some(deg),
+        (MNode::Rad { e, .. }, 1) => Some(e),
+        (MNode::Nary { sub, .. }, 0) => Some(sub),
+        (MNode::Nary { sup, .. }, 1) => Some(sup),
+        (MNode::Nary { e, .. }, 2) => Some(e),
+        (MNode::Delim { elems, .. }, i) => elems.get(i),
+        (MNode::Func { name, .. }, 0) => Some(name),
+        (MNode::Func { e, .. }, 1) => Some(e),
+        (MNode::Lim { e, .. }, 0) => Some(e),
+        (MNode::Lim { lim, .. }, 1) => Some(lim),
+        (
+            MNode::Acc { e, .. }
+            | MNode::Bar { e, .. }
+            | MNode::BorderBox { e, .. }
+            | MNode::Boxed { e }
+            | MNode::GroupChr { e, .. }
+            | MNode::Phant { e, .. },
+            0,
+        ) => Some(e),
+        (MNode::EqArr { rows }, i) => rows.get(i),
+        (MNode::Matrix { rows, .. }, i) => rows.iter().flatten().nth(i),
+        _ => None,
+    }
+}
+
+pub fn child_mut(n: &mut MNode, i: usize) -> Option<&mut Arg> {
+    match (n, i) {
+        (MNode::Frac { num, .. }, 0) => Some(num),
+        (MNode::Frac { den, .. }, 1) => Some(den),
+        (MNode::Script { base, .. }, 0) => Some(base),
+        (MNode::Script { sub, .. }, 1) => Some(sub),
+        (MNode::Script { sup, .. }, 2) => Some(sup),
+        (MNode::Rad { deg, .. }, 0) => Some(deg),
+        (MNode::Rad { e, .. }, 1) => Some(e),
+        (MNode::Nary { sub, .. }, 0) => Some(sub),
+        (MNode::Nary { sup, .. }, 1) => Some(sup),
+        (MNode::Nary { e, .. }, 2) => Some(e),
+        (MNode::Delim { elems, .. }, i) => elems.get_mut(i),
+        (MNode::Func { name, .. }, 0) => Some(name),
+        (MNode::Func { e, .. }, 1) => Some(e),
+        (MNode::Lim { e, .. }, 0) => Some(e),
+        (MNode::Lim { lim, .. }, 1) => Some(lim),
+        (
+            MNode::Acc { e, .. }
+            | MNode::Bar { e, .. }
+            | MNode::BorderBox { e, .. }
+            | MNode::Boxed { e }
+            | MNode::GroupChr { e, .. }
+            | MNode::Phant { e, .. },
+            0,
+        ) => Some(e),
+        (MNode::EqArr { rows }, i) => rows.get_mut(i),
+        (MNode::Matrix { rows, .. }, i) => rows.iter_mut().flatten().nth(i),
+        _ => None,
+    }
+}
+
+/// Call `f` on every run in the tree.
+pub fn each_run_mut(root: &mut Arg, f: &mut dyn FnMut(&mut MRun)) {
+    fn go(a: &mut Arg, f: &mut dyn FnMut(&mut MRun), depth: usize) {
+        if depth > MAX_DEPTH {
+            return;
+        }
+        for n in a.iter_mut() {
+            if let MNode::Run(r) = n {
+                f(r);
+            }
+            for c in 0..child_count(n) {
+                if let Some(ch) = child_mut(n, c) {
+                    go(ch, f, depth + 1);
+                }
+            }
+        }
+    }
+    go(root, f, 0);
+}