Theory related to **Lambda** types of Prove-It expressions. A **Lambda** has *parameter(s)* and a *body* and represents a mapping of the *parameter(s)* to the *body*. For example, $(x, y) \mapsto x + y$ represents a mapping of $x$ and $y$ into $x + y$; the application of this mapping to the arguments $5$ and $2$ yields $5+2$.

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```
import proveit
%theory # toggles between interactive and static modes
```