Yield Token Position

pub struct YieldTokenPosition {
    /// Link to address that owns this position
    pub owner: Pubkey,

    /// Link to vault that manages the YT
    pub vault: Pubkey,

    /// Track the YT balance of the user here
    pub yt_balance: u64,

    /// Tracker for interest earned (paid in SY)
    pub interest: YieldTokenTracker,

    /// Tracker for emissions earned (paid in emission tokens)
    pub emissions: Vec<YieldTokenTracker>,
}

pub struct YieldTokenTracker {
    /// The index is the per-share value of the SY token
    /// Note that the YT balance must be converted to the equivalent SY balance
    pub last_seen_index: Number,

    /// Staged tokens that may be withdrawn
    pub staged: u64,
}

earned_sy = yt_balance * (1/last_seen_rate - 1/final_rate)
scaled_sy = earned_sy * (final_rate / current_rate)  // if current > final