theorem Th17:
  t is compound implies ex o,p st t = o-term p
  proof reconsider d = t as Term of S,X by MSAFREE4:42;
    assume t.{} in [:the carrier' of S, {the carrier of S}:];
    then consider a,b being object such that
A1: a in the carrier' of S & b in {the carrier of S} & d.{} = [a,b]
    by ZFMISC_1:def 2;
    reconsider a as OperSymbol of S by A1;
    b = the carrier of S by A1,TARSKI:def 1;
    then consider p being ArgumentSeq of Sym(a,X) such that
A2: d = [a, the carrier of S]-tree p by A1,MSATERM:10;
    Free(S,X) = FreeMSA X by MSAFREE3:31;
    then reconsider p as Element of Args(a,Free(S,X)) by INSTALG1:1;
    take a,p;
    thus thesis by A2;
  end;
