R Under development (unstable) (2024-02-14 r85901 ucrt) -- "Unsuffered Consequences"
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> library(cpr)
> 
> x     <- seq(1e-5, 5.99999, length.out = 100)
> bmat  <- bsplines(x, iknots = c(1, 1.5, 2.3, 4, 4.5), bknots = c(0, 6))
> theta <- matrix(c(1, 0, 3.5, 4.2, 3.7, -0.5, -0.7, 2, 1.5), ncol = 1)
> cp0   <- cp(bmat, theta)
> cp1   <- insert_a_knot(x = cp0, xi_prime = 3)
> 
> stopifnot(inherits(cp1, "cpr_cp"))
> 
> stopifnot(isTRUE(
+     all.equal(
+       cp1$cp
+       ,
+       structure(list(xi_star = c(0, 0.333333333333333, 0.833333333333333, 1.6, 2.26666666666667, 3.1, 3.83333333333333, 4.83333333333333, 5.5, 6),
+                      theta = c(1, 0, 3.5, 4.2, 3.86666666666667, 1.6, -0.537837837837838, -0.7, 2, 1.5)),
+                 class = "data.frame",
+                 row.names = c(NA, -10L))
+       )
+     )
+ )
> 
> proc.time()
   user  system elapsed 
   0.25    0.06    0.29