Define a function (header level) which takes a parameter level. The header function should return the similar basic header function provided that n is between one and six. If n is outside this interval, we want header to return the identity function of one parameter.
It means that ((header 3) "Header text") is equal to (h3 "Header text") and that ((h 0) "Header text") is just "Header text".
Hint: Arrange the header functions in a list, and let header select the appropriate header function from this list.
Define a variant of header which returns a native header function if it receives a single parameter (level), and which returns the value, such as, ((header 3) "Header text"), if it receives both a level parameter and a header text string.