MathCases

class NeuronVoltage(Scene):
    def construct(self):

        cases = MathCases(
            CaseLine("0,",       "t <    0"),
            CaseLine("te^{-t},", "t \geq 0")
        )

        tex = MathTex("v(t) = ", cases)
        self.add(tex)

        tex.set_color(LIGHT_BROWN)
        cases[1].output.set_color(GREEN)
        cases[1].condition.set_color(BLUE)

class PredicatePartition(Scene):
    def construct(self):

        tex = MathTex("f(x) =", MathCases(
            CaseLine(r"b,     ", r"\text{even}(x)"),
            CaseLine(r"\neg b,", r"\text{odd}(x)"),
            CaseLine(r"?,",      r"\text{otherwise}"),
        ))
        self.add(tex)

        cases = tex[1]
        cases[1].condition.set_color(ORANGE)
        cases[2].output.set_color(ORANGE)

        self.play(TransformInStages.progress(tex))

        cases[1].condition = cases[2].condition
        cases.lines = [ cases[0], cases[1] ]

        self.play(TransformInStages.progress(tex))