Incorrect assertions used in the compiler's RascalPrimitive.

[Anastassija]

The following examples make the compiler crash due to assertions:

lrel[int] l = []; l<0>;
rel[int]  r = {}; r<0>;

The respective assertions are used in the Rascal primitives: lrel_field_project, rel_field_project, @PaulKlint, could you, please, check whether they should go or be changed?