### Breaking

[
  Datatype breaking
  Funcon   broken
  Funcon   finalise-breaking
  Funcon   break
  Funcon   handle-break
]


Meta-variables
  T <: values


Datatype
  breaking ::= broken
/*
  `broken` is a reason for abrupt termination.
*/


Funcon
  finalise-breaking(X:=>T) : =>T|null-type
   ~> finalise-abrupting(X)
/*
  `finalise-breaking(X)` handles abrupt termination of `X` due to executing
  `break`.
*/


Funcon
  break : =>empty-type
   ~> abrupt(broken)
/*
  `break` abruptly terminates all enclosing computations until it is handled.
*/


Funcon
  handle-break(_:=>null-type) : =>null-type
/*
  `handle-break(X)` terminates normally when `X` terminates abruptly for the
  reason `broken`.
*/
Rule
                X --abrupted( )-> X'
  ------------------------------------------------
  handle-break(X) --abrupted( )-> handle-break(X')
Rule
                X --abrupted(broken)-> _
  ---------------------------------------
  handle-break(X) --abrupted( )-> null-value
Rule
                X --abrupted(V:~breaking)-> X'
  ------------------------------------------------
  handle-break(X) --abrupted(V)-> handle-break(X')
Rule
  handle-break(null-value) ~> null-value