'Add a child to the tree t.  If child is a flat tree (a list), make all
in list children of t.  Warning: if t has no children, but child does
and child isNil then you can decide it is ok to move children to t via
t.children = child.children; i.e., without copying the array.  Just
make sure that this is consistent with have the user will build
ASTs.'

def addChild(self, tree, child):

    if ((tree is not None) and (child is not None)):
        tree.addChild(child)
