let get_env ?default var =
try
let varval = Sys.getenv var in
debug "ENV %s=%S" var varval ; varval
with Not_found ->
debug "ENV %s not set." var ;
match default with
| Some varval ->
debug "ENV %s default(%S)" var varval ; varval
| None ->
debug "ENV %s undefined." var ;
raise Not_found