--- The "agenda design pattern" ---
--- (c) EM/1415

open RelCalc

sig A {			-- the agenda owner
    R : set B	-- her/his tasks of type B
  , S : set C	-- her/his tasks of type C
}

sig B { f : one D }	-- slot of tasks of type B

sig C { g : one D }	-- slot of tasks of type C

sig D {}		-- where theoverlapping is checked 

pred inv{	-- no tasks of whatever type overlap
   img[R] & ker[f] in iden -- tasks in R don't overlap
   img[S] & ker[g] in iden -- tasks in R don't overlap
   ~S . R & g . ~f in iden -- mutual overlapping forbidden
}

run { inv }

-- special case: R = S and f = g, then img[R] & ker[f] in iden is enough