A quantified statement: forall x in S : P, exists x in S : P and
exists! x in S : P. A binder, like a set builder: the name is bound throughout
the body and the set, and the statement is a function of what else the body mentions.
The quantification set is mandatory, as Sullivan and Mackey's proofs book insists — a
statement is quantified over something, and forall x : x^2 >= 0 is true of
the reals and false of the complex numbers.
https://github.com/asc-community/AngouriMath/issues/1409
https://github.com/asc-community/AngouriMath/issues/225