Consider the word concordance program given in Example 12 of Appendix G. The runtime environment for this program needs to properly manage the block structure. The program executes by first declaring the doc variable. It then calls the concordance method. The first two lines of this method introduce the variables lineNumber and concord. These local variables only exist in the runtime environment while the concordance method is executing. Upon return from that method, these variables are removed from the environment. Continuing, the concord variable is added to the environment. Model this application as a List in which each entry is a Map, as described in the preceding exercise. When we enter a new block, a new Map is appended to the List, and when we exit the block, that Map is removed. As variables are introduced into the environment, they are recorded in the most recent Map from the List. Provide the following methods:

(a) add Block: that introduces a new block in the environment

(b) remove Block: that mimics exiting a block

(c) add: that introduces a new variable into the environment

(d) lookup: that obtains the value for a given variable.