Write a class MathCourse which inherits from the existing class Course:
open class Course(val code: String?, val title: String?)
MathCourse should have an additional String field area, which is the branch of math that course
pertains to. This value should be read-only.
MathCourse's default constructor should accept parameters for the code, title, and area
in that order.
MathCourse should also override toString() so that it returns a String of the format:
"(code): (title) is in the (area) branch of math."
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