function checkCompiling(schema, root, baseId) {
  /* jshint validthis: true */
  var index = compIndex.call(this, schema, root, baseId);
  if (index >= 0) return { index: index, compiling: true };
  index = this._compilations.length;
  this._compilations[index] = {
    schema: schema,
    root: root,
    baseId: baseId
  };
  return { index: index, compiling: false };
}
Checks if the schema is currently compiled
@this   Ajv
@param  {Object} schema schema to compile
@param  {Object} root root object
@param  {String} baseId base schema ID
@return {Object} object with properties "index" (compilation index) and "compiling" (boolean)