Clodagh
Playing chess with pigeons
OP, I understand perfectly that you took the pup on to give her a temporary home whilst she looks for a long-term home and that you wouldn't be able to perform the same service for more dogs if you kept each one to crossed your path.
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I acknowledge that you said this, but I think fosterers serve a vital role and helping this little dog to become a settled member of someones family is more the point than Thistle keeping every dog that she does a mercy dash to save. And the rescues rule may not be one we agree with, but is the rules. If the pup comes into season while she is there she will be moved to another foster home.