networks: Move sbf/state? to a better place.

networks.rkt

@@ -960,13 +960,6 @@
 ;;; applied to states in an unambiguous ways.
 (struct tbf/state (weights threshold) #:transparent)
 
-;;; A sign Boolean function (SBF) is a TBF whose threshold is 0.
-(define sbf/state? (and/c tbf/state? (λ (tbf) (zero? (tbf/state-θ tbf)))))
-
-(module+ test
-  (test-case "sbf/state?"
-    (check-true (sbf/state? (tbf/state #hash((a . -1) (b . 1)) 0)))))
-
 ;;; Shortcuts for acessing fields of a state/tbf.
 (define tbf/state-w tbf/state-weights)
 (define tbf/state-θ tbf/state-threshold)
@@ -982,6 +975,13 @@
     (check-equal? (tbf/state-w f) #hash((x1 . 1) (x2 . 1)))
     (check-equal? (tbf/state-θ f) 1)))
 
+;;; A sign Boolean function (SBF) is a TBF whose threshold is 0.
+(define sbf/state? (and/c tbf/state? (λ (tbf) (zero? (tbf/state-θ tbf)))))
+
+(module+ test
+  (test-case "sbf/state?"
+    (check-true (sbf/state? (tbf/state #hash((a . -1) (b . 1)) 0)))))
+
 ;;; Applies a state TBF to its inputs.
 ;;;
 ;;; Applying a TBF consists in multiplying the weights by the