Description: Exportation inference. (This theorem used to be labeled "exp" but was
changed to "ex" so as not to conflict with the math token "exp", per the
June 2006 Metamath spec change.) A translation of natural deduction
rule -> I ( -> introduction), see natded . (Contributed by NM, 3-Jan-1993)(Proof shortened by Eric Schmidt, 22-Dec-2006)