Def reducible(a) == b,c:. (b ~ 1) & (c ~ 1) & a = bc

is mentioned by

Def atomic(a) == a = 0 & (a ~ 1) & reducible(a)

[atomic]

Try larger context: StandardLIB IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html