build/transaction.go
func (b *TransactionBuilder) Sign(signers ...string) (result TransactionEnvelopeBuilder) {
	result.Mutate(b)

	for _, s := range signers {
		result.Mutate(Sign{s})
	}

	return
}
func (b *TransactionBuilder) Sign(signers ...string) (result TransactionEnvelopeBuilder) {
	result.Mutate(b)

	for _, s := range signers {
		result.Mutate(Sign{s})
	}

	return
}
// Sign returns an new TransactionEnvelopeBuilder using this builder's
// transaction as the basis and with signatures of that transaction from the
// provided Signers.