This module lets you defer actions at run-time that will take place
when the control flow returns into an upper scope. Currently, you can:

  -  hook an upper scope end with "reap" ;
  -  localize variables, array/hash values or deletions of elements
     in higher contexts with respectively "localize", "localize_elem"
     and "localize_delete" ;
  -  return values immediately to an upper level with "unwind", "yield"
     and "leave" ;
  -  gather information about an upper context with "want_at"
     and "context_info" ;
  -  execute a subroutine in the setting of an upper subroutine stack
     frame with "uplevel" ;
  -  uniquely identify contexts with "uid" and "validate_uid".
