theorem Th61:
  for a being negative expression of C, an_Adj C
  ex a9 being positive expression of C, an_Adj C
  st a = (non_op C)term a9 & Non a = a9
proof
  let a be negative expression of C, an_Adj C;
  consider a9 being expression of C, an_Adj C such that
A1: a9 is positive and
A2: a = (non_op C)term a9 by Def38;
A3: a = [non_op, the carrier of C]-tree<*a9*> by A2,Th43;
  reconsider a9 as positive expression of C, an_Adj C by A1;
  take a9;
  len <*a9*> = 1 by FINSEQ_1:40;
  then a|<* 0*> = <*a9*>.(0+1) by A3,TREES_4:def 4
    .= a9;
  hence thesis by A2,Def36;
end;
