Def as[m..n] == firstn(n-m;nth_tl(m;as))

is mentioned by

Thm* as:T List, i:{0...||as||}, j:{i...||as||}. ||as[i..j]|| = j-i

[length_segment]

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