def positive_to_range(exp, a, b):
    """
    Mapping a positive parameter 'exp' from the interval <0, \\infty) to the interval <a,b).
    Suitable e.g. to generate meaningful porosity from a variable with lognormal distribution.
    :param exp: A positive parameter. (LogNormal distribution.)
    :param a, b: Range interval.
    """
    return b * (1 - (b - a) / (b + (b - a) * exp))
