Return all condition keys in self.conditions as a list sorted
        suitable for print or write to a file. If prefix is given return
        only the ones prefixed with prefix.

def sorted_conkeys(self, prefix=None):
        """Return all condition keys in self.conditions as a list sorted
        suitable for print or write to a file. If prefix is given return
        only the ones prefixed with prefix."""

        # Make for defined and sorted output:
        conkeys = []
        for cond in _COND_PREFIXES:
            conkeys += sorted([key for key in self.conditions
                               if key.startswith(cond)], key=self.cond_int)
        if not prefix:
            return conkeys
        return [key for key in conkeys if key.startswith(prefix)]