Calculating closure of functional dependencies

closure.py

fds = {
    ('A', 'B'),
    ('A', 'G'),
    ('B', 'C'),
    ('C', 'D'),
    ('BC', 'E'),
    ('CG', 'A'),
}
start = 'B'

output = {start}

while True:
    used = set()
    for lhs, rhs in fds:
        lhsset = set(lhs)
        if lhsset <= output:
            output.add(rhs)
            used.add((lhs, rhs))
    fds -= used
    if not (fds and used):
        break

print(sorted(list(output)))