code: private int validated(final int idx) {
          if (idx < 0) {
              throw new IllegalArgumentException(
                  String.format("Index %d must be >= 0.", idx)
              );
          }
          if (idx >= this.all.size()) {
              throw new IllegalArgumentException(
                  String.format(
                      "Trying to return index %d from a list of %d passes",
                      idx,
                      this.all.size()
                  )
              );
          }
          return idx;
      }
docs: Validate index.
  @param idx Index of a Pass whose Identity to return on successful
  {@link PsAll#enter(Request)}
  @return Validated index