I haven't actually dug up the source, so I'm not sure if the gaps here are related, but GC Rota apparently claimed that one reason the umbral calculus remained mysterious for as long as it did was about ambiguities with `=`.
> Rota later stated that much confusion resulted from the failure to distinguish between three equivalence relations that occur frequently in this topic, all of which were denoted by "=".
> Rota later stated that much confusion resulted from the failure to distinguish between three equivalence relations that occur frequently in this topic, all of which were denoted by "=".
https://en.wikipedia.org/wiki/Umbral_calculus#The_modern_umb...