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