option
(
int
)
index_p (
(
α
bool
)
 
p
, 
list
(
α
)
 
l)
Search for an element in a list.This function searches the first occurrence of any element matching a given predicate
and returns its index.
p
A function deciding whether the element is the one we are looking for.
l
The list in which to find.
returns
{none} if no acceptable element doesn't appear in the list or {some = i}if the first occurrence of x appears at index i.

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