Talk about input constraint trick

[erhant]

When a single input does not really contribute to the constraint itself, but you would still like it to be bound to the proof, you may need to constraint it in a dummy way.

For example, suppose you want the proof to include the private input a but that input is not used in the circuit at all. If you simply keep it as a input, Circom may remove it from the constraint system as it is not used. You may want to introduce a dummy signal such as:

signal tmp <== a * a;

This will ensure a is part of a non-linear constraint.