  In previous works, the universal mapping class group was taken to be the
group PPSL(2,Z) of all piecewise PSL(2,Z) homeomorphisms of the unit circle S^1
with finitely many breakpoints among the rational points, and in fact, the
Thompson group T is isomorphic to PPSL(2,Z). The new spin mapping class group
P(SL(2,Z)) is defined to be all piecewise-constant maps from S^1 to SL(2,Z)
which projectivize to an element of PPSL(2,Z). We compute a finite presentation
of PPSL(2,Z) from basic principles of general position as an orbifold
fundamental group. The orbifold deck group of the spin cover is explicitly
computed here, from which follows also a finite presentation of P(SL(2,Z)).
This is our main new achievement. Certain commutator relations in P(SL(2,Z))
seem to organize according to root lattices, which would be a novel
development. We naturally wonder what is the automorphism group of P(SL(2,Z))
and speculate that it is a large sporadic group. There is a companion paper to
this one which explains the topological background from first principles,
proves that the group studied here using combinatorial group theory is indeed
P(SL(2,Z)).
