why there is no constraints in some Fp operation?

[readygo67]

in fields/fp/mod.rs, some operations has no constraints, such as add and mul_constant .

while some operation like mul applied constraints.

As a user, I have to check each operation has constraints or not, it's very confusing. Can arkworks add uniform constraints in its implementation?

[Pratyush]

Adding constraints for these operations would greatly blow up the constraint count for many circuits. Generally, in most arithmetic circuit systems, addition gates and multiply-by-constant gates tend to have much lower cost than multiplication gates, and r1cs-std reflects that