WIP Solution for #94

[parisholley]

I'm hitting my head on a wall on a way to implement a condition method to resolve #94 .. I feel like I'm running into some variation of microsoft/TypeScript#30853

I thought maybe we could be creative with a "is" check function:

function isArray<U>(state: any): state is ReadonlyArray<U> {
    return state.keys && typeof state.keys[0] === 'number';
}

or casting, but can't find a good way to get the check to go away

[codecov-commenter]

Codecov Report

Merging #95 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files           1        1           
  Lines         480      480           
  Branches      132      132           
=======================================
  Hits          436      436           
  Misses         44       44           

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