What needs to be debugged in this Plait $($Racket$)$ code? Return fixed code, it must pass test cases.

Define Types:

digit: Represents a single digit $(0-9).$

three$-$digits: A sequence of $3$ digits.

four$-$digits: A sequence of $4$ digits.

area$-$code: Can be either $787$ or $939.$

phone$-$number: Matches the format xxx$-$xxx$-$xxxx$.$

Validation Function:

Use a regular expression to check if the input string follows the phone number format with the specified area codes.

Final Code:

#lang plait

; Define a type for a single digit $(0-9)$

type digit $=0|1|2|3|4|5|6|7|8|9$

; Define a type for three digits $($a sequence of $3$ digits$)$

type three$-$digits $=[$digit digit digit$]$

; Define a type for four digits $($a sequence of $4$ digits$)$

type four$-$digits $=[$digit digit digit digit$]$

; Define a type for the area code $(787$ or $939)$

type area$-$code $=787|939$

; Define a type for the phone number $($format: xxx$-$xxx$-$xxxx$)$

type phone$-$number $=[$area$-$code $"-"$ three$-$digits $"-"$ four$-$digits$]$

; Function to validate the phone number

$($: validate$-$phone$-$number $($String $->$ Boolean$))$

$($define $($validate$-$phone$-$number phone$)$

$($define phone$-$pattern $($regexp $"^\backslash \backslash (787\backslash \backslash |939\backslash \backslash )-[0-9]\{3\}-[0-9]\{4\}$$$"))$

$($regexp$-$match? phone$-$pattern phone$))$

; Test cases

$($validate$-$phone$-$number $"787-123-4567")$ ; should return #t $($true$)$

$($validate$-$phone$-$number $"939-987-6543")$ ; should return #t $($true$)$

$($validate$-$phone$-$number $"800-123-4567")$ ; should return #f $($false$)$

$($validate$-$phone$-$number $"787-12-4567")$ ; should return #f $($false$)$