Function expression 

pub fn expression(ast: &Ast, index: ExprRef) -> String

One expression, written back out.

Public because the binder names an unaliased target after the text that produced it, and for a window call that text is this one: SELECT sum(x) OVER (ORDER BY x) has a column called sum(x) OVER (ORDER BY x) and getting there any other way would be a second deparser.