Only one posting with null amount allowed per transaction

[tbm]

Consider:

2010-01-01 open Assets:Cash
2010-01-01 open Equity:Opening-balance

2020-01-01 * "Opening balance: cash"
  Assets:Cash                                               0.10 EUR
  Assets:Cash                                               1.00 GBP
  Equity:Opening-balance

This leads to:

account Assets:Cash

account Equity:Opening-balance

2020-01-01 * Opening balance: cash
  Assets:Cash                                                      0.10 EUR
  Equity:Opening-balance
  Assets:Cash                                                      1.00 GBP
  Equity:Opening-balance

which leads to:

While parsing file "/home/tbm/src/tbm/beancount2ledger/x", line 11:
Error: Only one posting with null amount allowed per transaction

[tbm]

This was sort of introduced in commit 27dcd7e

Without it, we have:

2020-01-01 * Opening balance: cash
  Assets:Cash                                                      0.10 EUR
  Equity:Opening-balance                                          -0.10 EUR
  Assets:Cash                                                      1.00 GBP
  Equity:Opening-balance                                          -1.00 GBP

so the problem doesn't show up.