You can pass in property names and values that are then expanded in the source files. `expand()` allows for more than basic token substitution as the embedded expressions are full-blown Groovy expressions.

NOTE: Specifying the character set when reading and writing the file is good practice.

// reference to that substitution refers to the template parameter
// with the same index in the currently active template, not to
// whatever the template parameter would be expanded to here.  We sort
// this out in substitution and simplify.
func (st *state) templateParam() AST {
	off := st.off
	str := st.str
	st.checkChar('T')

	level := 0