--------------------------------------------------
lines 6-50 of file: at_cascade/get_fit_children.py
--------------------------------------------------

{xrst_begin get_fit_children}
{xrst_spell
  len
}

Determine the Set of Nodes to Fit
#################################

Prototype
*********
{xrst_literal ,
    # BEGIN_DEF, # END_DEF
    # BEGIN_RETURN, # END_RETURN
}

root_node_id
************
This is the node_id in the node_table for the
:ref:`glossary@root_node`.

fit_goal_set
************
This is a ``set`` with elements of type ``int``
specifying the node_id for each element of the
:ref:`glossary@fit_goal_set` or :ref:`fit_goal_table-name` .

node_table
**********
This is python list of python dictionaries
containing the dismod_at node table.

fit_children
************
The return value *fit_children* is a python list of python sets.

For each *node_id*
*fit_children* [ *node_id* ] is a the set of node ids that
are children of *node_id* and must be fit in order to fit
all the nodes in *fit_goal_set*.

Note that there must be at least one *node_id* in *fit_goal_set* ,
that is not an ancestor of any node in *fit_goal_set* and hence
len( *fit_children* [ *node_id* ] ) = 0 .

{xrst_end get_fit_children}