_ne: used to denote a concept specific to network expansion problems_directed: used to denote a concept specific to implementations where direction of flow is pre-determined._ls: used to denote a concept specific to implementations where production and consumption are variables._on_off: used to denote a concept where there are either-or choices, such is valve operations or binary direction choices.
p: pressure squaredpd_min: minimum pressure squared difference (computed value)pd_max: maximum pressure squared difference (computed value)
f: mass flowfl: mass flow consumptionfg: mass flow production