Function for negating the operation of a conversion that proves a formula to
be either true or false.
DESCRIPTION
This function negates the operation of a conversion that proves a formula to
be either true or false. For example, if conv proves "t" to be equal to
"T" then NEGATE_CONV conv will prove "~t" to be "F".
FAILURE
Fails if the application of the conversion to the negation of the formula does
not yield either "T" or "F".