def explanation_param(self, expr):
specifier = "<STR_LIT>" + str(next(self.variable_counter))<EOL>self.explanation_specifiers[specifier] = expr<EOL>return "<STR_LIT>" + specifier + "<STR_LIT>"<EOL>
Return a new named %-formatting placeholder for expr.

        This creates a %-formatting placeholder for expr in the
        current formatting context, e.g. ``%(py0)s``.  The placeholder
        and expr are placed in the current format context so that it
        can be used on the next call to .pop_format_context().