replace negate conditional with remove conditional

[hcoles]

Removes negate conditionals from the default set and replaces it with remove conditional (false). Remove conditional (true) is added to the STRONGER set.

This leaves both sets creating the same number of mutants, but as remove conditional is more stable than negate conditional a stronger test suite is required to kill the mutants.

Remove conditional logically subsumes negate conditional. We can see this with a truth table.

The != operator gives

true true -> false
true false -> true
false true -> true
false false -> false

Its negation (==) gives

true true -> true
true false -> false
false true -> false
false false -> true

where as replacing it with false gives

true true -> false
true false -> false
false true -> false
false false -> false

So, if the test suite provided only true,true and false,false as inputs, the mutant would be killed for negation, but survive for removal.