You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ast.go
    path of file: ./repos/aresdb/query/expr
    
    the code of the file until where you have to start completion: // Modifications Copyright (c) 2017-2018 Uber Technologies, Inc.
// Copyright (c) 2013-2016 Errplane Inc.
//
// Per
    
func CloneExpr(expr Expr) Expr {
	if expr == nil {
		return nil
	}
	switch expr := expr.(type) {
	case *UnaryExpr:
		return &UnaryExpr{Op: expr.Op, Expr: CloneExpr(expr.Expr)}
	case *BinaryExpr:
		return &BinaryExpr{Op: expr.Op, LHS: CloneExpr(expr.LHS), RHS: CloneExpr(expr.RHS)}
	case *BooleanLiteral:
		return &BooleanLiteral{Val: expr.Val}
	case *Call:
		args := make([]Expr, len(expr.Args))
		for i, arg := range expr.Args {
			args[i] = CloneExpr(arg)
		}
		return &Call{Name: expr.Name, Args: args}
	case *Case:
		conds := make([]WhenThen, len(expr.WhenThens))
		for i, cond := range expr.WhenThens {
			conds[i].When = CloneExpr(cond.When)
			conds[i].Then = CloneExpr(cond.Then)
		}
		var elce Expr
		if expr.Else != nil {
			elce = CloneExpr(expr.Else)
		}
		return &Case{WhenThens: conds, Else: elce}
	case *Distinct:
		return &Distinct{Val: expr.Val}
	case *NumberLiteral:
		return &NumberLiteral{Val: expr.Val, Expr: expr.Expr}
	case *ParenExpr:
		return &ParenExpr{Expr: CloneExpr(expr.Expr)}
	case *StringLiteral:
		return &StringLiteral{Val: expr.Val}
	case *GeopointLiteral:
		return &GeopointLiteral{Val: expr.Val}
	case *VarRef:
		return &VarRef{Val: expr.Val}
	case *Wildcard:
		return &Wildcard{}
	}
	panic("unreachable")
}