aa = 10;
ba = 11;
ca = 12;
da = 13;
ea = 14;
fa = fa + 1 : 14;

ab = 10;
bb = 11;
cb = 12;
db = 13;
eb = 14;

fb = sum( key ?a of . & set( cb, db ) & key f? except set( da, ba ) );

set s = set( key ?a of . );

n = sum( S ) - product( S );

real y = <nil> : 0;
z = ( y = 0 ) + for( real x, s ) { y = y + x };
