The conversion P_FUN_EQ_CONV embodies the fact that two functions are equal
precisely when they give the same results for all values to which they can be
applied. For any paired variable structure "p" and equation "f = g",
where p is of type ty1 and f and g are functions of type ty1->ty2,
a call to P_FUN_EQ_CONV "p" "f = g" returns the theorem:
|- (f = g) = (!p. f p = g p)