*procedure
**
READN
      To read a number from the current input stream.
      n := readn ()
      Characters are read from the currently selected  input
stream until one is found that is not a space, a newline, or
a tab character.  If it is'nt a digit `+' or `-', the result
is zero, the result2 global is set to -1,  the  character is
`unread' using unrdch.
      Any subsequent contiguous digits form the number.  The
number is terminated by the first non-digit encountered, and
this character is unread.  When readn succeeds,  the  result
is the number read, and the result2 global is set to 0.
      Note that a plus or minus sign which is  not  followed
by a digit will be read as zero.  No check is made for over-
flow while reading the number.


