Modular composition on representatives is evaluation in the quotient.
This is the bridge a computed fact has to cross to become a statement about
field elements: composeModMonic f b g = 0 is decide-able, and says exactly
that f vanishes at the class of b.
The composeModMonicImpl spelling of
Hex.FpPoly.Quotient.eval_reduce_eq_reduce_composeModMonic, for callers
whose computation ran through the compiled Horner loop.
Evaluating a polynomial at the quotient class of X returns its quotient
class.
This is the quotient evaluation map's defining property, stated for the project-side evaluator and executable reduction used by Conway compatibility.
A vanishing modular composition says the polynomial has the class of b as a
root.
The decide-able left-hand side and the field-level right-hand side, in one
step.