Suppose, I have two enums, Foo and A, and I need to convert specific patterns of Foo variants in a Vec<Foo> into A replacing that pattern of variants by an FA

enum Foo {
    F1(i64, i64, i64),
    F2(u64, i64),
    F3(i64, u64),
    FA(A)

}

enum A {
    Bar(u64, i64, u64),
    Baz(u64, i64, i64, u64),
}

For instance:
If a Foo::F2(x, y) is followed by a Foo::F3(z, w), combine them into A::Bar(x, y, w) if y == z

Right now, I have something like a state machine which keeps track of state while parsing.
But I need to do this for a lots of enum variants and that would get repetitive.