Parse one word.
Parse() returns parsed word as a structure.
Match phrase 'if word then print'.
Add if-expression structure.
Remove superfluous Word structure.
Fix test failure message (IfExpression does not have string() method yet).
Drop 'if' and 'then' constants from if-expression.
Add non-terminal name and colon.
Refactor: rename test.
Change from IfExpression to Sentence.
Remove IfExpression (replaced previously with Sentence).