code: func (c *Converter) RegisterUntypedConversionFunc(a, b interface{}, fn ConversionFunc) error {
  	return c.conversionFuncs.AddUntyped(a, b, fn)
  }
docs: // RegisterUntypedConversionFunc registers a function that converts between a and b by passing objects of those
  // types to the provided function. The function *must* accept objects of a and b - this machinery will not enforce
  // any other guarantee.