It has long been accepted that the illegality of part of a contract does not necessarily make the entire contract unenforceable. However, it is difficult to predict in a given case whether or not the court will hold that recovery may be had upon the lawful part of the contract. It is often said that such recovery will be allowed when the illegal portion of the contract can be clearly separated from the lawful part, but .even when stated in such broad terms - so broad in fact that it is of little help in solving the problem - the rule is, perhaps, not without exception.