lactose(in) + H(+)(in) <=> lactose(out) + H(+)(out)
lactose(in) + h(+)(in) <=> lactose(out) + h(+)(out)
lactose(in) + H(+)(in) <=> lactose(out) + H(+)(out)
lactose(in) + h(+)(in) <=> lactose(out) + h(+)(out)