# Returns a pretty-printable version of self
	#
	#     assert "( ( sp 12.3 ) \"DQString\")".to_sexp.as(SExp).pretty_to_s == "(\n\t(\n\t\tsp\n\t\t12.30\n\t)\n\t\"DQString\"\n)"
	fun pretty_to_s: String do return recurse_to_s(0)