Iteratively undischarges antecedents in a chain of implications.
DESCRIBE
A |- t1 ==> ... ==> tn ==> t
------------------------------ UNDISCH_ALL
A, t1, ..., tn |- t
Note that UNDISCH_ALL treats "~u" as "u ==> F".
FAILURE
Unlike UNDISCH, UNDISCH_ALL will,
when called on something other than an implication or negation,
return its argument unchanged rather than failing.
COMMENTS
Identical terms which are repeated in A, "t1", ..., "tn" will
not be duplicated in the hypotheses of the resulting theorem.
However, if two or more alpha-equivalent terms appear in A, "t1", ..., "tn",
then each distinct term will appear in the result.