Function parse_inductive 

pub fn parse_inductive(input: &str) -> Result<Command, ParseError>

Parse “A [Name] is either…” into Command::Inductive

Supports:

• A Bool is either Yes or No.
• A Nat is either: Zero. a Successor with pred: Nat.
• A List of (T: Type) is either: Empty. a Node with head: T and tail: List of T.