xwhile
======

.. py:function:: t3toolbox.backend.common.xwhile(cond, body, init_state, use_jit = False)

   .. code-block:: python

      def xwhile(
              cond,                  # state -> 0-d bool (eager) / traced bool (jit): loop while True
              body,                  # state -> state (a pytree of the SAME structure and leaf shapes)
              init_state,            # the loop-carried state (pytree)
              use_jit: bool = False, # True + jax state -> jax.lax.while_loop; else a Python while loop
      ):


   Data-dependent ``while`` with the numpy / eager-jax / jit dispatch -- the ``xscan`` precedent for a
   ``while``. With ``use_jit`` and a jax state it compiles via ``jax.lax.while_loop``; otherwise (numpy,
   eager jax, or jax not installed) it runs ``while bool(cond(state)): state = body(state)``, so it works
   on every backend and **silently falls back to eager** when jit is unavailable. Write ``cond``/``body``
   backend-agnostically (``cond`` returns a 0-d boolean; ``body`` uses ``xnp.where``, not Python branches)
   so the SAME pair drives both paths.