solve_xkcd135b <- function(h=0.001) {
    # start positions
    start_rtop <- c(0, sqrt(20^2-10^2))
    start_rright <- c(10, 0)
    start_rleft <- c(-10,0)
    start_me <- c(0, sqrt(3)/6*20)

    # survival time, given angle phi and stepsize h
    pos_me <- function(t, phi) start_me + c(sin(phi)*6*t, cos(phi)*6*t)
    velo_rtop <- function(t) pmin(4*t, 10) # wounded top raptor
    velo_rapt <- function(t) pmin(4*t, 25) # left & right
    dist_rapt <- function(from, to, velo) integrate(velo, from, to)$value
    surv_time <- function(phi, h, pos0, velo) {
        curr <- pos0
        t <- 0
        repeat {
            t <- t + h
            old <- curr
            direction <- c(pos_me(t-h, phi)-old)
            len <- dist_rapt(t-h, t, velo)
            if(dist(rbind(pos_me(t, phi), old))