type substitutions []AST

// add adds a new substitution candidate.
func (subs *substitutions) add(a AST) {
	*subs = append(*subs, a)
}

// subAST maps standard substitution codes to the corresponding AST.
var subAST = map[byte]AST{
	't': &Name{Name: "std"},
	'a': &Qualified{Scope: &Name{Name: "std"}, Name: &Name{Name: "allocator"}},