before_burn
Extendable function that allows the developer to add more functionality to the burn function, before the burn happens.
This allows, for example, to check that any given condition is present before burning is allowed.
If this function throws an exception, the asset will not be burned.
Throws if any extension of this function throws.
Parameters
account
the account that is going to lose the burned tokens
asset
the asset that is going to be burned
amount
the amount of tokens that is going to be burned