code: func NewArrayType(len int64, elem Type) *ArrayType {
  	ts, ok := arrayTypes[len]
  	if !ok {
  		ts = make(map[Type]*ArrayType)
  		arrayTypes[len] = ts
  	}
  	t, ok := ts[elem]
  	if !ok {
  		t = &ArrayType{commonType{}, len, elem}
  		ts[elem] = t
  	}
  	return t
  }
docs: // Two array types are identical if they have identical element types
  // and the same array length.