UnaryExpression concept

Expressions with a single argument. A UnaryExpression is almost a AdaptableUnaryFunction but an Expression is not Assignable.

In addition to those of Expression:

Argument type X::argument_type The type of the argument in the expression

`X`	A type that is a model of UnaryExpression
`x`	Object of type `X`
`a`	Object of type `X::argument_type`

In addition to those of Expression:

Name	Expression	Type requirements	Return type
Constructor	X x(a)		`X&`
Argument	x.argument()		X::argument_type

In addition to those of Expression:

Name	Expression	Precondition	Semantics	Postcondition
Constructor	X x(a);
Argument	x.argument()