I am reading the book titled "An Introduction to Statistical Learning with Applications in R" by James et al. On page 326, we perform cross-validation to determine the optimal level of tree complexity (for a classification tree). Here, you can find an extract from the provided R-code. As you can notice one of the values of $k$ (which is actually the tuning parameter $\alpha$ for cost-complexity pruning) equals $-\infty$. I was wondering, how can one obtain $-\infty$ for the cost complexity parameter?
Thanks in advance.