Further Examples: Unevaluated
Unevaluated stops Plus from evaluating its arguments so the result is Plus. Unevaluated is transparent to Head.
Here the arguments are evaluated so the result is no longer Plus.
The algebraic expression remains unevaluated so terms with head Integer, Rational, or Real are returned.
Here the algebraic expression evaluates to a complex number, so there are no terms left with the specified heads.
See also the Further Examples for Function.