Introduced conditions for nonnegative integers and for property lists.
Introduced conditions for nonnegative integers and for property lists.

Replaced most calls to check-type by manual checks and signals of more
specific type-error conditions.

Fixed some typos.

clasp-developers/SICL,vtomole/SICL,vtomole/SICL,vtomole/SICL,clasp-developers/SICL,clasp-developers/SICL,vtomole/SICL,clasp-developers/SICL