fix dependency (closes #1)

csp/constraint.rkt

@@ -1,5 +1,5 @@
 #lang racket/base
-(require racket/class racket/bool sugar/container sugar/list sugar/debug racket/list "helper.rkt" "variable.rkt")
+(require racket/class racket/bool sugar/unstable/container sugar/list sugar/debug racket/list "helper.rkt" "variable.rkt")
 (provide (all-defined-out))
 
 (define constraint%

csp/problem.rkt

@@ -1,5 +1,5 @@
 #lang racket/base
-(require racket/class sugar/container sugar/debug racket/contract racket/match racket/generator racket/list)
+(require racket/class sugar/unstable/container sugar/debug racket/contract racket/match racket/generator racket/list)
 (require "domain.rkt" "helper.rkt" "constraint.rkt" "solver.rkt")
 (provide (all-defined-out))
 

csp/solver.rkt

@@ -1,5 +1,5 @@
 #lang racket/base
-(require racket/class sugar/container sugar/debug racket/list 
+(require racket/class sugar/unstable/container sugar/debug racket/list 
          racket/bool racket/generator racket/match "helper.rkt")
 (provide (all-defined-out))