Section FsetPartitions.
Variables T I : choiceType.
Implicit Types (x y z : T) (A B D X : {fset T}) (P Q : {fset {fset T}}).
Implicit Types (J : pred I) (F : I -> {fset T}).
Definition fcover P := (\bigcup_(B <- P) B)%fset.
Definition trivIfset P := (\sum_(B <- P) #|` B|)%N == #|` fcover P|.

Lemma leq_card_fsetU A B :
((#|` A `|` B|)%fset <= #|` A| + #|` B| ?= iff [disjoint A & B]%fset)%N.