def tail(n: BigInt): BigInt = if (n <= 0) n else tail(n-1)

def sum(n: BigInt): BigInt = if (n <= 0) n else n + sum(n-1)

def silly(n: BigInt, sumsofar: BigInt): BigInt = if (n <= 0) sumsofar else silly(n-1, sumsofar+n)