generalized-Rabin syntax

Instead of

    acc-name: generalized-Rabin 3 2
    Acceptance: 7 (F0&I1&I2&I3)|(F4&I5&I6)

Joachim suggested we use

    acc-name: generalized-Rabin 2 3 2     /* note the leading 2 */
    Acceptance: 7 (F0&I1&I2&I3)|(F4&I5&I6)

Specifying the number of generalized pairs makes this closer to other acceptance names, and also helps parsing (in the same way as we give the number of atomic propositions before listing them on the `AP:` line).