According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern H--SF--. That is, seven letter words with 1st letter H and 4th letter S and 5th letter F. In alphabetical order, they are:

hatsful

The letter that have parallel line in it are H,I,F

lim as h->0 of (f(x+h) - f(x))/h

or

lim as x->a of (f(x) - f(a))/(x - a)

H-F

A function f(x) is not differentiable at x=a if:

lim h-->0 [f(a+h)-f(a)] / h does not exist.