Syntax: IF (expr) { list1 } <CR>
or      IF (expr) { list1 } ELSE { list2 }

   If `expr' is true, execute list1, otherwise list2 (if present).
Note the compulsory carriage return if the ELSE is omitted

   See also SET, CONNECT, HISTOGRAM, and POINTS for conditional
assignment and plotting, and the ?: operator under arithmetic.