where δ * ≡ t * s2 /t s1 and ≡ s 0 1 /s * 1 . If δ * 1, then we have three fast variables, c 1 , s 2 and c 2 , and one slow variable s 1 . This implies s 1 ≈ s 0 1 until c 1 , s 2 and c 2 reach their saturation values. After c 1 , s 2 and c 2 reach their threshold values, the complete system can be approximated (to leading order) as a differentialalgebraic equation (DAE):