def bond_length(rs, deriv=<NUM_LIT:0>):
return _bond_transform(rs, _bond_length_low, deriv)<EOL>
Compute the distance between the two points rs[0] and rs[1]

       Arguments:
        | ``rs``  --  two numpy array with three elements
        | ``deriv``  --  the derivatives to be computed: 0, 1 or 2 [default=0]

       When derivatives are computed a tuple with a single result is returned