Charlie, a beginner student of cryptography tries to design an original encryption scheme (warning: do not try this at home). One step is mixing two streams of hexadecimal digits (0 to 9, A to F). The digits of each stream must appear in the same order in the output, but may be arbitrarily mixed with each other; such a mix is called a proper mix. In his attempt to mix things up well, Charlie might have coded the algorithm improperly. Given two streams of m and n digits respectively, and an output of m and n digits, Required:
Determine whether the output is a proper mix of the two input streams or not?