code: def _merge_expressions(self, other):
          
          new_inputs = tuple(set(self.inputs).union(other.inputs))
          new_self_expr = self._rebind_variables(new_inputs)
          new_other_expr = other._rebind_variables(new_inputs)
          return new_self_expr, new_other_expr, new_inputs
docs: Merge the inputs of two NumericalExpressions into a single input tuple,
          rewriting their respective string expressions to make input names
          resolve correctly.
  
          Returns a tuple of (new_self_expr, new_other_expr, new_inputs)