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52: Mainline

In Segment 18, you learned that this book introduces many templatelike, general-purpose program patterns, called idioms, which you can fill in to suit your own specific purposes. In these patterns, each place to be filled in is identified by a box that contains a description of the item to be inserted, such as this.

The following, for example, is the pattern that you use when you want to display the answer produced by an expression in the transcript:

Transcript show: expression printString 

When you fill in such a pattern, replacing descriptions with specific instances, you are said to instantiate the pattern.