The structure of an expression

Expressions are combinations of variable references, function calls, and constant values. They are combined with operators such as "+", "||", and "!".

Operators come in three basic classes: unary, binary, and tertiary. Unary operators perform an operation on one operand, binaries on two, and tertiaries on three. Let's look at each in turn...

Unary: A typical unary operator is the C logical-NOT operator, "!", which takes the logical-NOT of its operand. The C-S tree for this looks like:
```
	-------
	|     |
	|  !  |--->NULL
	|     |
	-------
	   |
	   |
	  \|/
	---*---
	|     |
	| opr |--->NULL
	|     |
	-------
  
```

Binary: A typical binary operator is the C addition operator, "+", which adds its two operands based on type. The C-S tree for this looks like:


	-------
	|     |
	|  +  |--->NULL
	|     |
	-------
	   |
	   |
	  \|/
	---*----	--------	
	|      |	|      |	
	| opr1 |------->| opr2 |	
	|      |	|      |	
	--------	--------

Trinary: A typical trinary operator is the C trinary operator, "?:", which evaluates its first operand, then evaluates the second operand if the first was non-zero, or the third operand if the first was zero. The C-S tree for this looks like:
```
	-------
	|     |
	|  +  |--->NULL
	|     |
	-------
	   |
	   |
	  \|/
	---*----	--------	--------	
	|      |	|      |	|      |	
	| opr1 |------->| opr2 |------->| opr3 |	
	|      |	|      |	|      |	
	--------	--------	--------	
```



Note that, in general, each of these operands may actually be an arbitrarily
large tree, which must be evaluated down to a single value before being used by
the operation.


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