A scheme question,please help me

Use high order functions to define a mintree(TreeList) which returns a
new
BSetT with all the minimum elements from the TreelistTreelist is a
list of
BSetT. I have already solved most part of this question such as change
tree
to a set list and flatten all the set list. Since I am not familiar
with high
order function,I do not use map or apply to get the list.
For example,if the function is getmin,I need to use the (getmin '((1 2
3) (3 4 5)(6 7 8)(2 3 4)....) to get (1 3 6 2....) and then I can use
SetToTree to get
the mintree. I really appreciate your help.