FIRSTN_CONV : conv
STRUCTURE
listLib
SYNOPSIS
Computes by inference the result of taking the initial n elements of a list.
DESCRIPTION
For any object language list of the form “[x0;...x(n-k);...;x(n-1)]” , the result of evaluating 
   FIRSTN_CONV “FIRSTN k [x0;...x(n-k);...;x(n-1)]”
is the theorem 
   |- FIRSTN k [x0;...;x(n-k);...;x(n-1)] = [x0;...;x(n-k)]

FAILURE
FIRSTN_CONV tm fails if tm is not of the form described above, or k is greater than the length of the list.
HOL  Kananaskis-14