SemiBoxedExpression:
    | number
    | string
    | Decimal
    | Complex
    | MathJsonNumber
    | MathJsonString
    | MathJsonSymbol
    | MathJsonFunction
    | MathJsonDictionary
    | SemiBoxedExpression[]
    | BoxedExpression

A semi boxed expression is an MathJSON expression which can include some boxed terms.

This is convenient when creating new expressions from portions of an existing BoxedExpression while avoiding unboxing and reboxing.