I hope someone who has a copy will tell us what it says.
Well I checked my own copy yesterday evening and in fact they don't define a partial function
because in the triple (F,A,B) they require that A = domain F. Well...
Metamath has a definition of a partial function (but it is not (yet) ordinary mathematics.)
http://us2.metamath.org:88/mpegif/elpmg.html
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FL