Function decrement 

pub fn decrement(key: &str, delta: i64) -> Result<i64>

Atomically decrement a numeric value

If the key doesn’t exist, creates it with -delta as the initial value. This is equivalent to increment(key, -delta).

Arguments

• key - The key to decrement
• delta - The amount to subtract

Returns

• Ok(new_value) - The new value after decrement
• Err(StorageError) - Storage operation failed or value is not a valid i64

Example

// Decrement inventory
let remaining = storage::decrement("stock:item123", 1)?;
if remaining < 0 {
    println!("Out of stock!");
}