Challenge 7: Trading Fee Collected

Every swap has two related numbers:

• the user input amount
• the fee taken from that input

Task

Implement trading_fee(x_in).

Rules

• Fee rate = 30 basis points.
• 10_000 basis points = 100%.
• For x_in = 1000, fee = 1000 * 30 / 10_000 = 3.

Why it matters

A CPMM often needs to track the fee separately from the fee-adjusted input used in the quote formula.