<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def child(idx, dim, axis):
 """ Child node according to Bertran's notation. Parameters idx : int Index of the parent node. dim : int Dimensionality of the problem. axis : int Dimension direction to define a child. Must have `0<=axis<dim` Returns ------- out : int Index of child node with `out > idx`. Examples -------- 5 8 """

    idxm = multi_index(idx, dim)
    out = numpy.array(idxm) + 1*(numpy.eye(len(idxm))[axis])
    return single_index(out)