The function finds the congruence according to Brown & Gore (1994) between the three-letter Holland-codes given in argument a, which is the person code, and argument b, which is the environment code. The Index is (currently) only defined for three letters from the Holland code. The degree of congruence is output, according to its definition by Brown & Gore (1994), as a reciprocal value of a distance. This means, for example, that a value of '18' is the result for a perfect fit !