/sys$common/syshlp/helplib.hlb CC, Messages, NOSUBTRACT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        <Context> "<expression2>" cannot be subtracted from
                "<expression1>".

 Description    Because of their types, the two expressions cannot be
                used as the operands of the subtraction operator.
                Either both operands must be arithmetic type, or both
                operands must be pointers to qualified or unqualified
                versions of compatible object types, or the left
                operand must be a pointer type and the right operand
                must be an integral type.

 User Action    Modify the subtraction to use valid types.