Should scan another token and add it to the list, self.tokens,
        and add the restriction to self.restrictions

def scan(self, restrict):
        """
        Should scan another token and add it to the list, self.tokens,
        and add the restriction to self.restrictions
        """
        # Keep looking for a token, ignoring any in self.ignore
        while True:
            # Search the patterns for a match, with earlier
            # tokens in the list having preference
            best_pat = None
            best_pat_len = 0
            for p, regexp in self.patterns:
                # First check to see if we're restricting to this token
                if restrict and p not in restrict and p not in self.ignore:
                    continue
                m = regexp.match(self.input, self.pos)
                if m:
                    # We got a match
                    best_pat = p
                    best_pat_len = len(m.group(0))
                    break

            # If we didn't find anything, raise an error
            if best_pat is None:
                msg = "Bad Token"
                if restrict:
                    msg = "Trying to find one of " + ", ".join(restrict)
                raise SyntaxError(self.pos, msg)

            # If we found something that isn't to be ignored, return it
            if best_pat in self.ignore:
                # This token should be ignored ..
                self.pos += best_pat_len
            else:
                end_pos = self.pos + best_pat_len
                # Create a token with this data
                token = (
                    self.pos,
                    end_pos,
                    best_pat,
                    self.input[self.pos:end_pos]
                )
                self.pos = end_pos
                # Only add this token if it's not in the list
                # (to prevent looping)
                if not self.tokens or token != self.tokens[-1]:
                    self.tokens.append(token)
                    self.restrictions.append(restrict)
                    return 1
                break
        return 0