module Matching_block:sig..end
    get_matching_blocks a b returns a list of triples describing matching
    subsequences.
Each triple is of the form (i, j, n), and means that a <|> (i,i+n) = b <|> (j,j+n). The triples are monotonically increasing in i and in j.
The last triple is a dummy, (Array.length a, Array.length b, 0), and is the only triple with n=0.
    Example:
    get_matching_blocks |"a";"b";"x";"c";"d"| |"a";"b";"c";"d"|
    returns
    (0, 0, 2), (3, 2, 2), (5, 4, 0)
type |    | mine_start : | 
|    | other_start : | 
|    | length : |