M-p and M-n don't work in irb
ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-linux]
2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
irb is supposed to use readline and M-a, M-e are bound to beginning-of-line and end-of-line respectively. However, M-p and M-n don't have the same effect as up-arrow and down-arrow. Why is this?