checkNull = function(value) {
    if (value !== null) {
      // 550: where() clause includes an invalid predicate, can't compare
      // lf.Type.OBJECT to anything other than null.
      throw new lf.Exception(550);
    }
  }
/**
Builds a map associating evaluator types with the evaluator functions, for
the case of a column of type 'Object'.
NOTE: Only lf.eval.Type.EQ and NEQ are available for objects.

@return {!lf.structs.Map<!lf.eval.Type, !lf.eval.ComparisonFunction_>}
@private