<s>
A	O
human-to-computer	O
user	B-Application
interface	I-Application
is	O
said	O
to	O
be	O
"	O
reactive	O
"	O
if	O
it	O
has	O
the	O
following	O
characteristics	O
:	O
</s>
<s>
Gestures	O
can	O
be	O
keystrokes	O
,	O
mouse	B-Device
clicks	I-Device
,	O
menu	O
selections	O
,	O
or	O
more	O
esoteric	O
inputs	O
.	O
</s>
<s>
Did	O
I	O
just	O
overwrite	O
my	O
backup	B-Protocol
by	O
mistake	O
?	O
</s>
<s>
In	O
a	O
figure-drawing	O
program	B-Application
,	O
the	O
user	O
can	O
tell	O
whether	O
a	O
line	O
segment	O
is	O
composed	O
of	O
smaller	O
segments	O
.	O
</s>
<s>
A	O
program	B-Application
with	O
a	O
built-in	O
help	O
browser	O
is	O
not	O
reactive	O
if	O
its	O
content	O
is	O
just	O
a	O
collection	O
of	O
screen	B-Application
shots	I-Application
or	O
menu	O
item	O
labels	O
with	O
no	O
real	O
explanation	O
of	O
what	O
they	O
do	O
.	O
</s>
<s>
Reactivity	O
was	O
a	O
major	O
goal	O
in	O
the	O
early	O
user	B-Application
interface	I-Application
research	O
at	O
MIT	O
and	O
Xerox	O
PARC	O
.	O
</s>
<s>
A	O
computer	B-Application
program	I-Application
which	O
was	O
not	O
reactive	O
would	O
not	O
be	O
considered	O
user	O
friendly	O
no	O
matter	O
how	O
elaborate	O
its	O
presentation	O
.	O
</s>
<s>
Early	O
word-processing	B-General_Concept
programs	O
whose	O
on-screen	O
representations	O
look	O
nothing	O
like	O
their	O
printer	O
output	O
could	O
be	O
reactive	O
.	O
</s>
<s>
The	O
common	O
example	O
was	O
WordStar	B-Application
on	O
CP/M	B-Application
.	O
</s>
<s>
On-screen	O
,	O
it	O
looked	O
like	O
a	O
markup	O
language	O
in	O
a	O
character	O
cell	O
display	O
,	O
but	O
it	O
had	O
deep	O
built-in	O
help	O
which	O
was	O
always	O
available	O
from	O
an	O
on-screen	O
menu	O
bar	O
,	O
and	O
the	O
effect	O
of	O
each	O
keystroke	B-Application
was	O
obvious	O
.	O
</s>
