if a person has a key-chain that he found on shabbos with many keys, and one key which is his ignition key (kli shmelachto l’issur) and he wants to use one of his non-muktzeh keys, what should he do?
[On one hand, if the muktzehitem was ‘nail clipper’ for example he can pick up and use the whole keychain (minus the nail clip), since the nail clip is of secondary importance; however in the case above the ignition key is equally important as the other non muktzeh keys but cannot be used.]
It is permitted to move and make use of the key-chain on Shabbos.
As a keli shemelachto le’issur, it is permitted to move the ignition key for its use — for a permitted purpose — or for purpose of its place. When a person needs to use the entire key chain, he needs to move the ignition key, which is attached to the chain, for a permitted use: for use of the entire set of keys. This is therefore permitted on Shabbos.