contains
Does the specified flat list of phrases contain only statements?
TODO MvG - REVISIT to make this work sensibly. Probably only allow statements in a sequence/first-of-sequence, and have blocks hold an optional final expression.
Return
true if the list contains only statements, false otherwise.
Parameters
flat
A flattened list of statements.
result
The result type of the sequence. Use top if unconcerned about result type.