Are you sure about the first rule? It doesn't seem necessary according to https://oeis.org/A014575.
The problem is really fun.
Since I created the problem series and want the Functions I badge, I thought I'd provide the solution that incorporates vampire_factor and are_fangs. Not a great score, but readable!
Sorry, I mean the Magic Numbers badge.
Project Euler: Problem 8, Find largest product in a large string of numbers
What's Your BMI?
Television Screen Dimensions
Reverse the elements of an array
Product of two multivariate polynomials
Find all vampire fangs
Outer product of multiple vectors
Fangs of a vampire number
Negation the hard way
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