* 'i': check for "line" describing an integer in all its chars before end or \n
* (i.e. all other chars must be digits, except the first char, which may
* be '+' or '-'; a '+' or '-' without digits following is invalid)
* 'i': check for "line" describing an integer in all its chars before end or \n
* (i.e. all other chars must be digits, except the first char, which may
* be '+' or '-'; a '+' or '-' without digits following is invalid)