code: func WalkExprs(x interface{}, f func(*Expr) bool) {
  	vis := &GenericVisitor{func(x interface{}) bool {
  		if r, ok := x.(*Expr); ok {
  			return f(r)
  		}
  		return false
  	}}
  	Walk(vis, x)
  }
docs: // WalkExprs calls the function f on all expressions under x. If the function f
  // returns true, AST nodes under the last node will not be visited.