function [h,x]=span(f,x)
% span - divide sequence using a test function
%
% span :: (A->bool), seq(A) -> seq(A), seq(A).
%
% span(f,x) = takeWhile(f,x), dropWhile(f,x)
% Will not terminate if head segments turns out to be infinite.
[Y,x]=spanc(f,x);
h=cellseq(Y);
