The modulo operator is very common and useful to obtain the remainder of a division operation.
At first glance, the following code might look correct:

C_REAL($a) C_REAL($b) C_REAL($c) $a:=200000000001 $b:=10^10 $c:=$a%$b

While it might work for other data types, including integers and long integers, it might fail when using real values, as indicated in the 4D documentation. "The modulo "%" operator returns valid results with Integer and Long Integer expressions. To calculate the modulo of real values, you must use the Mod command."

In this case, the modulo "%" operator should be replaced by the Mod command:

$c:=Mod($a; $b)