In double-entry accounting, each type of account--asset, liability, revenue, expense, and owner's equity--has a normal balance of either debit or credit. Contra accounts are the exception to this rule. Assets normally carry a debit balance but can be reduced by related accounts known as contra assets.
The Snake is the universal symbol of evil. In Harry Potter this archetype holds true. Voldemortâs symbol is the snake and the symbol for Slytherine is also a snake. The students in the Slytherine House are all portrayed/really are, evil (example: Malfoy).
A doe, same as Lily Evans because he loved her so much. According to the character Remus Lupin, a witch or wizard's Patronus can change from a great shock or emotional upheaval (Harry Potter and the Half-Blood Prince, USA Scholastic hardcover edition, p340). Snape's Patronus may have changed because he was tormented by her death or it may always have been a Doe as he always loved her. In the chapter The Prince's Tale, in Harry Potter and the Deathly Hallows, Snape casts his doe Patronus in front of Dumbledore, 16 years after Lily's death.