f  g == <1of(f) @ filter(a.a  dom(f);1of(g)),a.f(a)?g(a)> 

latex

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clarification:

fpf-join(eq;f;g)
== <1of(f) @ filter(a.fpf-dom(eq; a; f);1of(g)),a.fpf-cap(f;eq;a;fpf-ap(g; eq; a))> 

latex

Definitions	f  g, as @ bs, filter(P;l), b, x  dom(f), 1of(t), f(x)?z, f(x)
FDL editor aliases	fpf-join