tt13

unix v7是最后一个广泛发布的研究型UNIX版本

.NH
Environments
.PP
As we mentioned, there is a potential problem
when going across a page boundary:
parameters like size and font
for a page title may well be different from those
in effect in the text when the page boundary occurs.
.UL troff
provides a very general way to deal with this and
similar situations.
There are three `environments',
each of which has independently settable versions of
many of the parameters associated with processing,
including size, font, line and title lengths,
fill/nofill mode, tab stops, and even partially collected lines.
Thus the titling problem may be readily solved by processing the main text
in one environment and titles in a separate one
with its own suitable parameters.
.PP
The command
.BD .ev\ n
shifts to environment
.BD n ;
.BD n
must be 0, 1 or 2.
The command
.BD .ev
with no argument returns to the
previous environment.
Environment names are maintained in a stack, so calls
for different environments may be nested and unwound consistently.
.PP
Suppose we say that the main text is processed in environment 0,
which is where 
.UL troff
begins by default.
Then we can modify the new page macro
.BD .NP
to process titles in environment 1 like this:
.P1 2
^de NP
^ev 1	\e" shift to new environment
^lt 6i	\e" set parameters here
^ft R
^ps 10
\&... any other processing ...
^ev	\e" return to previous environment
^^
.P2
It is also possible to initialize the parameters for an environment
outside the
.BD .NP
macro,
but the version shown keeps all the processing in one place
and is thus easier to understand and change.