function B=chunk(A,n,step)
% chunk - return chunk from inside matrix
%
% chunk :: 
%    [[N,M]]   ~'source array',
%    K:natural ~'index of chunk',
%    L:natural ~'chunk size'
% -> [[L,M]]   ~'K block of L rows'.

i=n*step+1;
B=A(i:i+step-1,:);


