Fixtype of binary exponents [C:6.4.4.2] [C:A.1.5].
This is a product type introduced by fty::defprod.
This corresponds to binary-exponent-part in the grammar in [C]. It consists of a prefix, an optional sign, and a list of (decimal) digits (which should be non-empty). The digits are decimal, not binary or hexadecimal; but the implicit base of the exponent is binary [C:6.4.4.2/3].