Support conditionals that stop evaluation early

[tangentstorm]

Add the control words and. and or. to allow things like

   if. a>:0 and. sentence_that_fails_if_a<0 do.

-- @HenryHRich 12:14, 6 November 2017 (UTC)

[tangentstorm]

+1 would love to see this!

[jip]

It seems to have already been implemented in addons:

   sentence_that_fails_if_a_lt_0=: 3 : 0
  assert y >: 0
  %: y
)
   sentence_that_fails_if_a_lt_0 9
3
   sentence_that_fails_if_a_lt_0 _9
|assertion failure: assert
|       assert y>:0
Press ENTER to inspect

   load 'misc/miscutils/langexten'
   sentence_that_fails_if_a_lt_0 and (>:&0) 9
3
   sentence_that_fails_if_a_lt_0 and (>:&0) _9
0
   NB. wrapper to demonstrate with (if.) control just like it was requested
   wrapper=: 3 : 0
if. sentence_that_fails_if_a_lt_0 and (>:&0) y do.
  echo 'condition is true'
  sentence_that_fails_if_a_lt_0 y
else.
  echo 'condition is false'
  _42
end.
)
   wrapper 9
condition is true
3
   wrapper _9
condition is false
_42

There is also or conjunction there. BTW, other variants: notAnd, orNot, andNot, notOr can be found in math/mt/mm.