if (not (isa (y, "infsupdec")))
y = infsupdec (y);
* Methods of class <code>infsupdec</code> as well as methods of class <code>infsup</code> that are not overridden by <code>infsupdec</code> must check parameters using the <code>isnai</code> function and return the NAI value if it is present. This will propagate NAI values through any function evaluations.
if (isnai (x))
result = x;