Growing points can join as well as split.
(define-growing-point ((diffusion p-fet Vdd) Q-id)
(material diffusion p-type)
(secrete (diffusion-short inhibitor) Q-id)
(tropism increasing Vdd-short)
(drop-metal-diffusion-contact)
(define-growing-point ((diffusion p-fet contact) Q-id)
(material diffusion p-type)
(secrete (diffusion-short inhibitor) Q-id)
(secrete (n-diffusion-attract attractor) Q-id)
(tropism increasing p-diffusion-attract Q-id)
(when (is-type? n-diffusion)
(drop-poly-diffusion-contact)
(start-growing-point (poly from-input-contact)