val letin_ex : MathQL.mqlvar -> Mathql_semantics.result -> Mathql_semantics.result
val letref_ex : MathQL.mqlvar -> Mathql_semantics.result
val letin_ex : MathQL.mqlvar -> Mathql_semantics.result -> Mathql_semantics.result
val letref_ex : MathQL.mqlvar -> Mathql_semantics.result