build/change_trust.go
func (b *ChangeTrustBuilder) Mutate(muts ...interface{}) {
	for _, m := range muts {
		var err error
		switch mut := m.(type) {
		case ChangeTrustMutator:
			err = mut.MutateChangeTrust(&b.CT)
		case OperationMutator:
			err = mut.MutateOperation(&b.O)
		default:
			err = errors.New("Mutator type not allowed")
		}

		if err != nil {
			b.Err = err
			return
		}
	}
}
func (b *ChangeTrustBuilder) Mutate(muts ...interface{}) {
	for _, m := range muts {
		var err error
		switch mut := m.(type) {
		case ChangeTrustMutator:
			err = mut.MutateChangeTrust(&b.CT)
		case OperationMutator:
			err = mut.MutateOperation(&b.O)
		default:
			err = errors.New("Mutator type not allowed")
		}

		if err != nil {
			b.Err = err
			return
		}
	}
}
// Mutate applies the provided mutators to this builder's payment or operation.