"set" quantification is allowed in syntax, but meaning isn't preserved

[tnelson]

sig A {}
test expect {
    higherOrderSome: {
        some X: set A | #X > 1
    } is sat
}

The above parses and runs without any error -- until the test fails. If we're going to permit Alloy-style higher-order some we need to preserve its semantics. Alternatively, we could give an error when someone tries to use Alloy-style some ... set ....