module Op: sig .. end
Open type and add combinators. Meant to be opened
type 'a job = 
val (@@>) : OpamProcess.command ->
       (OpamProcess.result -> 'a job) ->
       'a job
Stage a shell command with its continuation, eg:
        
          command "ls" ["-a"] @@> fun result ->
          if OpamProcess.is_success result then Done result.r_stdout
          else failwith "ls"
        
val (@@+) : 'a job ->
       ('a -> 'b job) -> 'b job
job1 @@+ fun r -> job2 appends the computation of tasks in job2 after
        job1
val (@@|) : 'a job -> ('a -> 'b) -> 'b job
job @@| f maps f on the results of job.
        Equivalent to job @@+ fun r -> Done (f r)