code: func (v *evalVisitor) popExpr() *ast.Expression {
  	if len(v.exprs) == 0 {
  		return nil
  	}
  
  	var result *ast.Expression
  	result, v.exprs = v.exprs[len(v.exprs)-1], v.exprs[:len(v.exprs)-1]
  
  	return result
  }
docs: // popExpr pops last expression from stack