lots-younger?: number(age1) number(age2) -> boolean. True if the
first person is lots younger than the second (15 years or more).
kinda-close? number(x) number(y) -> boolean. True if x and y are
“kinda close” to each other, which we define to be within 20.
is-green-circle?: image -> boolean. True if the image is a pure
green circle (of any radius).
is-expensive?: string(material) number(weight) -> boolean. True
if the material is diamond or the weight is more than 30.
15.3.4