digraph G {
    a1 [label = <Enter<br/>z = z_in<br/>x = x_in<br/>y = y_in>, shape = rect];
    a2 [label = "z += x"];
    a3 [label = "y++"];
    a4 [label = <z_out = z<br/>y_out = y<br/>Exit>, shape = rect];
    a1 -> a2 -> a3 -> a4

//    p [label=<x_in = a + b<br/>y_in = c<br/>f()<br/>c = y_out>,shape=rect];
    f_call [label="f()"]
    x_in [label="x_in = a + 1"]
    y_in [label="y_in = b"]
    z_in [label="z_in = z"]
    y_out [label="b = y_out"]
    z_out [label="z = z_out"]
    f_call -> {z_in x_in y_in y_out z_out};

    f_start [label="enter f", shape = rect];
    fx_in [label="x = x_in"];
    fy_in [label="y = y_in"];
    fz_in [label="z = z_in"];
    fy_out [label="y_out = y"];
    fz_out [label="z_out = z"];
    f_start -> {fz_in fx_in fy_in fy_out fz_out};
    f_call -> f_start [style=bold];
    y_in -> f_start [style=invis];
    {
        edge [style = dashed];
        z_in -> fz_in
        x_in -> fx_in
        y_in -> fy_in
        fy_out -> y_out
        fz_out -> z_out
    }
    invis [height=0.001,width=0.001,style=invis];
    invis2 [height=0.001,width=0.001,style=invis];
    {rank=same; x_in y_in y_out z_in z_out invis};
    {rank=same; fx_in fy_in invis2 fy_out fz_in fz_out};
    {edge [style=invis];
        z_in -> x_in -> y_in -> invis -> y_out -> z_out;
        fz_in -> fx_in -> fy_in -> invis2 -> fy_out -> fz_out;
    }

    {rank = max;
    zplus [label = "z += x"];
    yplus [label = "y++"];
    }
    f_start -> {zplus yplus};
    {
        edge [color = red];
        {fz_in fx_in} -> zplus;
        fy_in -> yplus;
        edge [constraint = false];
        zplus -> fz_out;
        yplus -> fy_out;
    }

    {
        edge [color = blue, constraint = false, style = bold];
        {z_in x_in} -> z_out;
        y_in -> y_out;
    }

}