sorry if I bother you again, but I tried to find some hints in the APL2 Language Reference Manual without luck.
In the Sullivan's paper, there's the reference to a "scan" operator quite fast more suited to be used with his multi precision package. This is its definition:
∇ z←(F scan)x;y
z←⊂y←⊃x ∆1:!(0=⍴x←1"x)/0 z,←⊂y←y F⊃x
!∆1
the "!" is the branch arrow.
Now the problem is with GNU APL I cannot define this operator, cause I don't know how to specify F is a function not a variable.