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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def calculate_sets(rules):
 """Calculate FOLLOW sets. Adapted from: http://lara.epfl.ch/w/cc09:algorithm_for_first_and_follow_sets"""

    symbols = {sym for rule in rules for sym in rule.expansion} | {rule.origin for rule in rules}

    # foreach grammar rule X ::= Y(1) ... Y(k)
    # if k=0 or {Y(1),...,Y(k)} subset of NULLABLE then
    #   NULLABLE = NULLABLE union {X}
    # for i = 1 to k
    #   if i=1 or {Y(1),...,Y(i-1)} subset of NULLABLE then
    #     FIRST(X) = FIRST(X) union FIRST(Y(i))
    #   for j = i+1 to k
    #     if i=k or {Y(i+1),...Y(k)} subset of NULLABLE then
    #       FOLLOW(Y(i)) = FOLLOW(Y(i)) union FOLLOW(X)
    #     if i+1=j or {Y(i+1),...,Y(j-1)} subset of NULLABLE then
    #       FOLLOW(Y(i)) = FOLLOW(Y(i)) union FIRST(Y(j))
    # until none of NULLABLE,FIRST,FOLLOW changed in last iteration

    NULLABLE = set()
    FIRST = {}
    FOLLOW = {}
    for sym in symbols:
        FIRST[sym]={sym} if sym.is_term else set()
        FOLLOW[sym]=set()

    # Calculate NULLABLE and FIRST
    changed = True
    while changed:
        changed = False

        for rule in rules:
            if set(rule.expansion) <= NULLABLE:
                if update_set(NULLABLE, {rule.origin}):
                    changed = True

            for i, sym in enumerate(rule.expansion):
                if set(rule.expansion[:i]) <= NULLABLE:
                    if update_set(FIRST[rule.origin], FIRST[sym]):
                        changed = True

    # Calculate FOLLOW
    changed = True
    while changed:
        changed = False

        for rule in rules:
            for i, sym in enumerate(rule.expansion):
                if i==len(rule.expansion)-1 or set(rule.expansion[i+1:]) <= NULLABLE:
                    if update_set(FOLLOW[sym], FOLLOW[rule.origin]):
                        changed = True

                for j in range(i+1, len(rule.expansion)):
                    if set(rule.expansion[i+1:j]) <= NULLABLE:
                        if update_set(FOLLOW[sym], FIRST[rule.expansion[j]]):
                            changed = True

    return FIRST, FOLLOW, NULLABLE