is mentioned by

Def arr1 @ arr2 == <arr1.l,arr2.u,(i.if i<arr1.u arr1[i] ; arr2[i] fi)>	[array_append]
Def a[++i] == <(a.l+i),(a.u+i),(j.a[(j-i)])>	[array_shift]

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