$title Onmulti operates like a replace (ONMULTI2,SEQ=24)
set i    / a, b, c /
parameter a / (a,b,c) = 10 /;

$onMulti

parameter a / b = 0, c = 20 /;

abort$(card(a) <>  2) "card(a) <>  2",a;
abort$(a('a')  <> 10) "a('a')  <> 10",a;
abort$(a('b')  <>  0) "a('b')  <>  0",a;
abort$(a('c')  <> 20) "a('c')  <> 20",a;