Bad error message for bad Clifford generators

[ecpeterson]

Generating a God table works when the generators are Clifford elements, but will later cause failure if those generators only generate a subgroup of the Clifford group.

Example we ran into:

{ "depth": 2, "qubits": 1, "gateset": ["PHASE(pi) 0", "H 0"] }

Here, PHASE(pi) is not a generator of the Clifford group. It should be PHASE(pi/2) or S to generate C_1.

Putting the above in a hash table and running (rb-post nil ht nil nil) repeatedly causes failure. It should fail! But it needs a better error message/detected earlier.

Possible solution: Return an error "No decomposition exists." or the like.

Reported by @stylewarning .