code: func (c *Converter) WithConversions(fns ConversionFuncs) *Converter {
  	copied := *c
  	copied.conversionFuncs = c.conversionFuncs.Merge(fns)
  	return &copied
  }
docs: // WithConversions returns a Converter that is a copy of c but with the additional
  // fns merged on top.