let x be object ; :: according to PRALG_1:def 12 :: thesis: ( not x in proj1 <%P,Q%> or <%P,Q%> . x is 1-sorted )
assume x in dom <%P,Q%> ; :: thesis: <%P,Q%> . x is 1-sorted
then ( x = 0 or x = 1 ) by TARSKI:def 2;
hence <%P,Q%> . x is 1-sorted ; :: thesis: verum