Should tt(uflow) be overridden?  The function tt(uflow) is called when it's
available to return the next character from the device. In practice this is
handled by tt(underflow), so there's probably little need for overriding
tt(uflow). But em(if) tt(uflow) is overridden then it must return the next
avaialble character and update tt(gptr()) to the next available
character. If the update isn't performed then the returned characters is
received twice by the stream. Here's a skeleton:
    verbinclude(-a examples/uflow.cc)

