• Recursive traversal of the AST • E.g. for a simple addition expression
	ExpTy transExp( Absyn.OpExp e ) {    switch (e.oper) {      case Absyn.OpExp.PLUS:        checkInt( transExp( e.left),            e.left.pos);        checkInt( transExp( e.right),            e.right.pos);        return new ExpTy( null, INT );    } }