156: Highlights

A Boolean expression is an expression that evaluates to true or false.
A block is a sequence of statements delimited by brackets. The value produced when a block is evaluated is the value produced by the final statement in the block.
If you want to evaluate an expression only when a Boolean expression evaluates to true, then use an ifTrue: message:
```
Boolean expression ifTrue: if-true block 
```
If you want to evaluate an expression only when another expression evaluates to false, then use an ifFalse: message:
```
Boolean expression ifFalse: if-false block 
```
If you want to evaluate one expression when a predicate expression evaluates to true, and a second expression, when the predicate expression evaluates to false, then use an ifTrue:ifFalse: or an ifFalse:ifTrue: message:
```
Boolean expression 
  ifTrue: if-true block 
  ifFalse: if-false block 
Boolean expression  
  ifFalse: if-false block 
  ifTrue: if-true block 
```
If you want to a method to answer with a value produced inside a block, then include an answer expression in that block.