Function solana_sdk::keyed_account::keyed_account_at_index

pub fn keyed_account_at_index<'a>(
    keyed_accounts: &'a [KeyedAccount<'_>], 
    index: usize
) -> Result<&'a KeyedAccount<'a>, InstructionError>

Return the KeyedAccount at the specified index or a NotEnoughAccountKeys error

Index zero starts at the chain of program accounts, followed by the instruction accounts.