func (this *Cover) Evaluate(item value.Value, context Context) (value.Value, error) {
var rv value.Value
switch item := item.(type) {
case value.AnnotatedValue:
rv = item.GetCover(this.text)
}
if rv == nil {
return value.MISSING_VALUE, errors.NewEvaluationError(nil, "cover("+this.text+")")
}
return rv, nil
}