code: function create (spec) {
    const accumulator = new Accumulator()
  
    accumulator.value = spec.value
    accumulator.context = spec.context
    accumulator.reducer = {
      spec: spec.context
    }
    accumulator.entityOverrides = merge({}, spec.entityOverrides)
    accumulator.locals = merge({}, spec.locals)
    accumulator.values = spec.values
    accumulator.trace = spec.trace
    accumulator.debug = debug
  
    return accumulator
  }
docs: creates new Accumulator based on spec
  @param  {Object} spec - acumulator spec
  @return {Source}