[ESS] A-M-r and A-M-s

[Manuel Cebrian]

In the (official, posted on this page) ESS manual there are two key
bindings A-M-r and A-M-s attached to two commands, commint-previous-
matching-input and commint-next-matching-input respectively.

¿Could anybody explain to me what is the 'A'? It becomes clear that the
'M' key is the meta key and the 'C' key is the control key.

Thank you in advance,

Manuel Cebrian