Loop variable problems cat - Error, invalid parameters for inline function

I have a loop that will map specific locations of a set of lists into a new set of lists.

Suppose
a1:=[1,2,3,4,5,6,7]
a2:=[9,8,7,6,5,4,3]
a3:=[4,5,4,5,4,5,4]
:
etc ...

for i from 1 to 3 do
  b||i:=map(a||i->a||i[4],a||i[6],a||i):  #map positions 4 and 6 from a into b
end do:

I get Error, invalid parameters for inline function.