it exists some errors in RealtimeDeque's check method

[onionyuyixi]

Hello! Your works is amazing and makes me cheerful when i do search some reasonable materials to aquire skills what《Purely Functional Data Structure》tells me . so I cloned the repository and am learning it by comparing the book with the codes and did some test for more precise understanding ,while finding some flaws i commit here . So if you had some spare time , I hope the work more complete sincerely.

error details as following image

and the pfd book says what the next image shows