<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate(self, cmd, messages=None):
 """Returns True if the given Command is valid, False otherwise. Validation error messages are appended to an optional messages array. """

        valid = True
        args  = [ arg for arg in cmd.args if arg is not None ]

        if self.nargs != len(args):
            valid = False
            if messages is not None:
                msg  = 'Expected %d arguments, but received %d.'
                messages.append(msg % (self.nargs, len(args)))

        for defn, value in zip(self.args, cmd.args):
            if value is None:
                valid = False
                if messages is not None:
                    messages.append('Argument "%s" is missing.' % defn.name)
            elif defn.validate(value, messages) is False:
                valid = False

        if len(cmd._unrecognized) > 0:
            valid = False
            if messages is not None:
                for name in cmd.unrecognized:
                    messages.append('Argument "%s" is unrecognized.' % name)

        return valid