proc tokenizeString(expression: string): seq[Token] =
    if len(expression) == 0:
        raise newException(ValueError, "Expression cannot be empty.")

    var
        tokens: seq[Token] = @[]
        startIndex: int = 0

    while startIndex < len(expression):
        endIndex = len(expression) - 1

        var foundToken: bool = false
        while endIndex >= startIndex && !foundToken:
            var token: Token = parseToken(expression[startIndex..endIndex])

            if token.tokenType != TokenType.Invalid:
                startIndex = endIndex + 1
                tokens.add(token)
                foundToken = true

            endIndex++

        if !foundToken: <-- Invalid Indentation Here
            raise newException(ValueError, "Expression has an invalid token at " & startIndex)

    return tokens