Return a list of all the symbols contained in this expression.
        Include recursively subexpressions symbols.
        This includes duplicates.

def get_symbols(self):
        """
        Return a list of all the symbols contained in this expression.
        Include recursively subexpressions symbols.
        This includes duplicates.
        """
        return [s if isinstance(s, Symbol) else s.args[0] for s in self.get_literals()]