Outline Exercise 2
Finish exercise sheet for Session 2
Merge branch 'master' of github.com:klauso/PLT2013
Move the remark on function names and variable names living in different namespaces later, in order to avoid wrong implication that the `eval` given right after the remark implements one common namespace for function names and variable names.  They still live in different namespaces because their the substitution for function names is still handled outside the \`subst\` for variable substitutions"