build/transaction.go
func (b *TransactionBuilder) Mutate(muts ...TransactionMutator) {
	if b.TX == nil {
		b.TX = &xdr.Transaction{}
	}

	for _, m := range muts {
		err := m.MutateTransaction(b)
		if err != nil {
			b.Err = err
			return
		}
	}
}
func (b *TransactionBuilder) Mutate(muts ...TransactionMutator) {
	if b.TX == nil {
		b.TX = &xdr.Transaction{}
	}

	for _, m := range muts {
		err := m.MutateTransaction(b)
		if err != nil {
			b.Err = err
			return
		}
	}
}
// Mutate applies the provided TransactionMutators to this builder's transaction