Instrumental (slit) function.
    B(x) = 1/γ , if |x| ≤ γ/2 & B(x) = 0, if |x| > γ/2,
    where γ is a slit width or the instrumental resolution.

def SLIT_RECTANGULAR(x,g):
    """
    Instrumental (slit) function.
    B(x) = 1/γ , if |x| ≤ γ/2 & B(x) = 0, if |x| > γ/2,
    where γ is a slit width or the instrumental resolution.
    """
    index_inner = abs(x) <= g/2
    index_outer = ~index_inner
    y = zeros(len(x))
    y[index_inner] = 1/g
    y[index_outer] = 0
    return y