+ (Cube _ i j k _ _) = cube
+ f = value_at fvs (i-1) j k
+ b = value_at fvs (i+1) j k
+ l = value_at fvs i (j-1) k
+ r = value_at fvs i (j+1) k
+ d = value_at fvs i j (k-1)
+ t = value_at fvs i j (k+1)
+ fl = value_at fvs (i-1) (j-1) k
+ fr = value_at fvs (i-1) (j+1) k
+ fd = value_at fvs (i-1) j (k-1)
+ ft = value_at fvs (i-1) j (k+1)
+ bl = value_at fvs (i+1) (j-1) k
+ br = value_at fvs (i+1) (j+1) k
+ bd = value_at fvs (i+1) j (k-1)
+ bt = value_at fvs (i+1) j (k+1)
+ ld = value_at fvs i (j-1) (k-1)
+ lt = value_at fvs i (j-1) (k+1)
+ rd = value_at fvs i (j+1) (k-1)
+ rt = value_at fvs i (j+1) (k+1)
+ fld = value_at fvs (i-1) (j-1) (k-1)
+ flt = value_at fvs (i-1) (j-1) (k+1)
+ frd = value_at fvs (i-1) (j+1) (k-1)
+ frt = value_at fvs (i-1) (j+1) (k+1)
+ bld = value_at fvs (i+1) (j-1) (k-1)
+ blt = value_at fvs (i+1) (j-1) (k+1)
+ brd = value_at fvs (i+1) (j+1) (k-1)
+ brt = value_at fvs (i+1) (j+1) (k+1)
+ int = value_at fvs i j k
+