This function checks whether the value in a LAMBDA is missing and returns TRUE or FALSE.
ISOMITTED(argument)
Specify a value you want to test, such as a LAMBDA parameter.
Returns TRUE or FALSE
ISOMITTED(a)
LAMBDA(x,y, IF(ISOMITTED(y),"Missing second argument",x+y))(1,)
This function is available in product version 15 or later.