i have a movieclip on stage having instance name 'a_mc'

there is another movieclip inside movieclip 'a_mc' placed on frame 20 of 'a_mc'
which has instance name 'b_mc'


how do i call movieclip 'b_mc' from my main timeline using action script 3


for eg i do this.. a_mc.b_mc.x = 500;

this is returning a error now
"TypeError: Error #1010: A term is undefined and has no properties.
at Untitled_fla::MainTimeline/frame1()"

this method only seems to work when i place 'b_mc' at first frame