Function builtin_cov 

fn builtin_cov(
    args: &[RValue],
    _named: &[(String, RValue)],
) -> Result<RValue, RError>

Sample covariance of two numeric vectors.

Computes sum((x - mean(x)) * (y - mean(y))) / (n - 1).

@param x numeric vector @param y numeric vector (same length as x) @return scalar double