See BP1061 for a version of this with only squares and which allows infinite nesting.
See BP71 for a Problem about counting levels of nesting.
Adjacent-numbered pages: BP1017 BP1018 BP1019 BP1020 BP1021  *  BP1023 BP1024 BP1025 BP1026 BP1027
easy, nice, precise, allsorted, traditional
varied_thickness_curves_drawing [smaller | same | bigger]
Aaron David Fairbanks
Based on BP30, "curve with exactly one self-crossing vs. curve without a self-crossing". This is just a slightly more general version.
Adjacent-numbered pages: BP1227 BP1228 BP1229 BP1230 BP1231  *  BP1233 BP1234 BP1235 BP1236 BP1237
EX6881
stub, notso, traditional
crosscurves [smaller | same | bigger]