def add_species_aliases(registry, ftype, alias_species, species):
    """
    This takes a field registry, a fluid type, and two species names.  
    The first species name is one you wish to alias to an existing species
    name.  For instance you might alias all "H_p0" fields to "H\\_" fields
    to indicate that "H\\_" fields are really just neutral hydrogen fields.
    This function registers field aliases for the density, number_density,
    mass, and fraction fields between the two species given in the arguments.
    """
    registry.alias((ftype, '%s_density' % alias_species), (ftype, 
        '%s_density' % species))
    registry.alias((ftype, '%s_fraction' % alias_species), (ftype, 
        '%s_fraction' % species))
    registry.alias((ftype, '%s_number_density' % alias_species), (ftype, 
        '%s_number_density' % species))
    registry.alias((ftype, '%s_mass' % alias_species), (ftype, '%s_mass' %
        species))
