Pseudocode for different ways nice programming by context could work:

My initial suggestion was this:
if c < 0: it.le else: it.ri
  if context == nil:
    context = n
    return
  it = context

Here you have a variable context that is automatically created in the same manner result is for functions.

Since then I learned about byAddr (formerly known as byRef), and if I'm correct about it is used it would look like this:

byAddr context = if c < 0: it.le else: it.ri
if context == nil:
  context = n
  return
it = context

There are three differences here:
 1) explicit use of byAddr which also has other uses and can be used by other means than if.
 2) the context variable name is programmer defined, not by convention, this makes the code longer,
    but also enables nesting of this concept
 3) there's no idented block, which I see as a disadvantage, because it signified where the context is
    relevant, for this block of code only and not for what follows, which is the idea of "context"
    But this is also a matter of taste, ..., could be good as well

There was also an idea by PMunch:

template withContext(check, a, b, body): untyped =
  if check:
    template context: untyped = a
    body
  else:
    template context: untyped = b
    body

(c < 0).withContext(it.le, it.ri):
  if context == nil:
    context = n
    return
  it = context

1) Here he have an indented block.
2) withContext is descriptive
3) But is there any chance this could become idiomatic Nim? I used repetitive code from Nim's docs to make the example, because I wanted to make the case that there's currently no idiomatic way to solve this.

Another idea for a special syntax I just had:

way: # block that defines the way in that something. that depends on a context dependend variable, should be done
  if context == nil:
    context = n
    return
  it = context

if c < 0: do it.le else: do it.ri # do initiates the way that something should be acted upon either it.le or it.ri here
# either it.le or it.ri became the context

1) this syntax is independend from if or any other structure, could work with case or anything else
2) do always sets context to the object given. Then executes last defined way block with that context.
3) new way blocks override old way blocks. There could also be way blocks inside other way blocks without an issue