VisitorMut add Expr

My intuition tells me that the VisitorMut walker should be able to add expressions to WHERE or init the selection Option to Some Expr in Statement::Query body::Select. Is there a quick example of this or is the VisitorMut capable of only traversing the AST nodes if they exist?

Start by reading the VisitorMut walker and the Statement::Query body::Select entry points mentioned in the issue. Determine whether mutation currently only visits existing AST nodes or can add a WHERE expression and initialize the selection Option. Done should include a decided behavior and coverage for the requested mutation, though no test file is named.

Written by the indexing model from the issue text.