Hi,
yes it is. Score was initially written in 1967 for PDP, then ported to S/360, in 1983 to PC DOS and only two years ago to Windows, where it is still beta.
Most users still use version 4.0 for DOS
http://en.wikipedia.org/wiki/SCORE_(software)
The input is can be manual or semi-graphical.
The manual is simply like lily: a text files (normally with extension .mus). Then the engine can load and transform the file in Postscript
The semigraphical. You first edit notes like in text files, but immediately after one line is finished it appears on the screen with a number over each note.
one can use the numbers are reference for expressions and dynamics:
e.g. : staccato from note 1 to 10, slur from 13 tp 14 and so on.
The input is a iterative process in 5 steps.
each line (a line can have multiple staves like, piano, quartets, orchestral,etc) of music goes to a file
at the and score will combine each file to a single one. (more or less like you do in C or Java per each class)
step1: notes
step2: rhythms
step3: marks
step4: beams
stes5: slurs
each item is separated by "/"
each line is ended with a ";"
here a tutorial:
http://www.ccarh.org/courses/253/handout/scoreinput/
tr/3 4/e4/m/cu/aj/m/f/d/m/bj/b/cu/d/e/m/c/aj/eu/m/cu/aj/m/f/e/m/ds/d/e/fs/d/m/e/r/ml;
this means:
tr = treble clef
3 4 = time signature 3 quarters
e4 = the first note "e" first line in treble clef
m = bar
cu = the note c (up) ==> like in lily by default use the closest note. "u" force the greater internal (a 6th up, instead of a 3rd down)
aj = the note a (down) ==> same story
etc
then follow the example in the link above
In my opinion SCORE is the only software able to write everything since it is basically a graphical software and not a sequencer (Finale, Sibelius, etc)
Lilypond is very similar as concept, but I have to say that it is very verbose and it needs still some releases to reach the same level, but I have great expectation from it.
SCORE is much more practical (and less logical). It is written with engravers in mind not musician. but this makes its interface much easier and faster.
Finally. I think score is the most used software by engravers. Schott use it for all contemporary scores. Other publishers use all the big ones (Sibelius, Finale) and Score for "important and expensive works"
Cheers
Ctesibius