EPTCS style changes spacing for fb arrows in proved system

[aubertc]

With EPTCS style, the \pr{fb} macro isn't quite right:

This can be fixed by replacing 5 by 4 in the following:

fbarrows/fbarrows.sty

Lines 154 to 158 in ddedec3

	{bf}{% backward & forward case needs some accomodation
	\mathrel{{\mapstochar\mkern-5mu \r{#1}[#2][#3]}}
	}
	{fb}{% forward & backward case needs some accomodation (same as above, it's an alias)
	\mathrel{{\mapstochar\mkern-5mu \r{#1}[#2][#3]}}

but not requiring adjustment based on the document's class would be an improvement.