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Threshold cryptosystem with a fixed *k*required share

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Threshold cryptosystem with a fixed *k*

I am trying to find an implementation or variant of a (k, n) threshold cryptosystem (as described here Shamir's Secret Sharing) where we can fix at least one of the k key parts, i.e., instead of ANY k key parts able to decrypt the ciphertext I want at least one of the k key parts to be a certain/fixed key part. So sort of a joint account cheque where k out of n signatures are required but the signature of the accountant should always be there. Any ideas?