A structured function is a function generated by connecting two
functions through a transition region.
This function calculates the value of a structured function.
# NOT RUN {x <- seq(1, 5, 0.1)
y <- c()
for (theta in x) y <- c(y, structural_function(theta, c(2, 3), log, sqrt, theta))
plot(x, y)
lines(x, log(x), col = "blue")
lines(x, sqrt(x), col = "red")
# }