def linear(x, param):
    """Linear function.
    
    f = param[0] * x + param[1] .
    
    Parameters:
        x (np.ndarray):    Positions at which to evaluate function.
        param (np.ndarray):    Necessary parameters.
        
    Returns:
        (np.ndarray):    Values of function at x.
        
    """
    return param[0] * x + param[1]
