added the function for free variables of a lambda-term, which is a tad more difficult to define than capture-avoiding substitution
added the function for free variables of a lambda-term, which is a
tad more difficult to define than capture-avoiding substitution

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle