@Nullable
  @Override
  public <R, C> R accept(ExpressionBiVisitor<R, C> visitor, @Nullable C context) {
    return visitor.visit(this, context);
  }
Converts current value to collection (if it is not already). If value
is iterable returns that collection (which is most likely ImmutableList already).

@return singleton list with current value or immutable list of values depending on type
of current value.
criteria/common/src/org/immutables/criteria/expression/Constant.java