\doc{Proofs by contradiction}
\ref{proof-by-contradiction}
\ref{contradiction}

Proofs by contradiction provide an indirect method of proof.  The command
\fq{prove F by contradiction} directs LP to prove a formula \f{F} by deriving
an \llink{inconsistency}{inconsistency} from LP's logical system supplemented
by the hypothesis \f{~F'}, where \f{F'} is the result of substituting new
\llink{constant}{constants} for the free variables in \f{F}.  (This hypothesis
is logically equivalent to the negation of \f{F} because introducing new
constants is equivalent to replacing the implicit universal quantifiers by
existential quantifiers.)  The name of the hypothesis has the form
\s{simpleId}\fq{ContraHyp.}\s{number}, where \s{simpleId} is the current value
of the \setlink{name-prefix} setting.
\p
The command \fq{resume by contradiction} directs LP to resume the proof of the
current conjecture by contradiction.
