code: function attemptBoolIndexing(selector, maxLen) {
    if (selector.type === exclusionProto.type) {
      var intIdxVector = attemptBoolIndexing(selector._selector, maxLen);
      return isUndefined(intIdxVector) ?
        undefined :
        excludeIntIndices(intIdxVector, maxLen);
    }
    if (Array.isArray(selector)) {
      selector = inferVectorDtype(selector.slice(), 'object');
    }
    if (selector.type === vectorProto.type && selector.dtype === 'boolean') {
      if (selector.size() !== maxLen) {
        throw new Error('inappropriate boolean indexer length (' +
          selector.size() + '); expected length to be ' + maxLen);
      }
      return selector.which();
    }
  }
docs: Performs boolean indexing if 'selector' is a boolean vector or array of the same length as maxLen or an exclusion wrapping such a vector or array. Returns undefined if 'selector' is inappropriate for boolean indexing; returns a vector of integer indices if boolean indexing resolves appropriately.