Coalesce()
Returns the first non NIL/NULL expression among its arguments.
Coalesce( <Expression,...> ) --> <Expression>
CoalesceEmpty()
Returns the first non empty expression among its arguments.
CoalesceEmpty( <Expression,...> ) --> <Expression>
IsNull()
Tests if an expression evaluates to NULL/NIL.
IsNull( <Expression> ) --> lIsNull IsNull( <Expression>, <ReplacementValue> ) --> xValue