---++ Syntax

<verbatim>
"if" Strategy "then" Strategy "else" Strategy "end" -> Strategy
</verbatim>

---++ Description

<verbatim>
<if s1 then s2 else s3 end> t
</verbatim>

If =s1= succeeds then strategy =s2= is applied. If =s1= fails, then =s3= is applied.

The result of condition is ignored. If the condition succeeds, then the result is

<verbatim>
<s2> t
</verbatim>

If the condition fails, then the result is

<verbatim>
<s3> t
</verbatim>

---++ Formatting

The if then else construct is usally formatted in the following way:

<verbatim>
if s1 then
  s2
else
  s3
end
</verbatim>

---++ Implementation

The if-then-else construct is sugar for a guarded left-choice. The
strategy 

<verbatim>
if s1 then s2 else s3 end
</verbatim>
translates into

<verbatim>
where(s1) < s2 + s3
</verbatim>