code: private void checkRefAgainstRefType(Ref ref, RefType refType) {
          if (ref != null && ref.getRefType() == refType) {
              throw new IllegalArgumentException("Invalid ref type passed as argument to checkout");
          }
      }
docs: This is just a test method for verifying that a given ref is not of refType provided as one of the parameters.
  
  @param ref     This could be branch, sha1 etc.
  @param refType This is the <code>RefType</code> which the ref should not match.