add override functions for accepting suppliers.

[XenoAmess]

it be needed as something like:
query several mongo collections one by one, etc.
t is useful, yeah, at lease my job needs it now...

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[XenoAmess]

@garydgregory please have a look and give some opinions.
once you aggree these changes, unit testes would be added...

just like our good old days, sigh.
but I'm no student anymore, and grow too busy to be very active...

[garydgregory]

Hello @XenoAmess

-1: I won't spend time reviewing this new code without unit tests. Instead, I'll spend my time writing this: Tests in PRs are important: They show intent, edge cases, and expected failures. A contributor must show the intended usage of the code, without an application, unit tests stand in for these call sites. Asking a maintainer (me) to spend their time when you did not spend your time doing the basics it not an equitable trade-off.

[XenoAmess]

Hello @XenoAmess

-1: I won't spend time reviewing this new code without unit tests. Instead, I'll spend my time writing this: Tests in PRs are important: They show intent, edge cases, and expected failures. A contributor must show the intended usage of the code, without an application, unit tests stand in for these call sites. Asking a maintainer (me) to spend their time when you did not spend your time doing the basics it not an equitable trade-off.

sounds reasonable.
My real situation, for which I need this pr, is for MongoClient produced iterables...
Though import MongoClient into this repo's unit test sounds crazy, I would make a mock one instead, and try to explain my expecting usage using codes.
Please wait for several hours.

[XenoAmess]

@garydgregory unit test added. please have a look