code: public final double at( long i ) {
      long x = i - (_start>0 ? _start : 0);
      if( 0 <= x && x < _len ) return at0((int)x);
      throw new ArrayIndexOutOfBoundsException(getClass().getSimpleName() + " " +_start+" <= "+i+" < "+(_start+_len));
    }
docs: Load a double value.  Returns Double.NaN if value is missing.
  <p>
  Loads from the 1-entry chunk cache, or misses-out.  This version uses
  absolute element numbers, but must convert them to chunk-relative indices
  - requiring a load from an aliasing local var, leading to lower quality
  JIT'd code (similar issue to using iterator objects).
  <p>
  Slightly slower than 'at80' since it range checks within a chunk.