Function substitute 

pub fn substitute(ast: &mut AstNode, bindings: &HashMap<String, AstNode>)

Replace variable references with sub-expressions.

Backs the GenApi <Constant> and named <Expression> elements, which let a formula name a literal or a reusable sub-formula:

<IntSwissKnife Name="Example">
  <pVariable Name="X">X</pVariable>
  <Constant Name="TEN">10</Constant>
  <Expression Name="XPLUS2">TEN + X</Expression>
  <Formula>TEN * XPLUS2</Formula>
</IntSwissKnife>

Substitution happens once, at build time, so evaluation stays a plain walk over the tree.