grade-school - first version
phone-number - 2nd version
robot-name - first version
fetch new exercise: leap
leap - first version
grade-school - version 2
leapyear - version 2
ETL - first version
ex 4.49 - impl
ex 4.49 - explanation on output
retrieve some new exercises
ex 4.49 - done
ex 4.50 - done
ex 4.54 - an example, and done
4.4 - logic programming

notes about
* imperative knowledge
* mathematical knowledge
* input & output of a program
4.4 - logic programming (cont.)
logic programming, unification
4.4 - logic programming. let's proceed
4.4.1 - a plan
4.4.1 - use gprolog