def check_reversibility(reaction):
    """
    Return:
     - *forward* if reaction is irreversible in forward direction
     - *backward* if reaction is irreversible in the rbackward (reverse) direction
     - *reversible* if the reaction is reversible
     - *blocked* if the reactions is blocked
    """
    if (reaction.lower_bound < 0) and (reaction.upper_bound == 0):
        return "backward"
    elif (reaction.lower_bound == 0) and (reaction.upper_bound > 0):
        return "forward"
    elif (reaction.lower_bound == 0) and (reaction.upper_bound == 0):
        return "blocked"
    else:
        return "reversible"