[Jprogramming] What did 11 : '+ [: x 0: = ]' mean?

[Dan Bron]

> 11 is no longer a valid right argument to : (Explicit), what 
> did it mean back then?

11 :  was to  1 :  as  13 :  is to  3 :  .

That is, 11 : (attempted) to produce a tacit version of the explicit adverb given in the string.  You could write an equivalent
tacit adverb today as  ([:`) (`(0 = ])) (`:6) (+`) (`:6)  .

-Dan

PS:  There was also  12 :  (convert to tacit conjunction) .  My memory is trying to tell me that  14 :  was also valid, and
converted strings to tacit dyads, but that doesn't make much sense.  Unless it was equivalent to  [: : (13 :)   ?