The function definition operator. X where X is numeric. All variables occurring in exp are presumed to be real valued. X where X is numeric. If X is not numeric then a simplified but equivalent form is returned.
TRUE means do the application.
The eqi are equations indicating substitutions to be made. For each equation, the right side will be substituted for the left in the expression exp. If the last argument to a part function is a list of indices then several subexpressions are picked out, each one corresponding to an index of the list. It is set during the execution of the function and thus may be referred to in the function itself as shown below. It returns the new value of exp.