correct compiling of block children to allow for maxima expressions #818

Job	Run time
test (8.2, master, pgsql, SBCL)	27m 30s
test (8.1, MOODLE_402_STABLE, pgsql, GCL)	22m 35s
test (8.0, MOODLE_402_STABLE, pgsql, SBCL)	26m 20s
test (7.4, MOODLE_401_STABLE, mariadb, GCL)	1h 8m 30s
test (7.4, MOODLE_400_STABLE, pgsql, SBCL)	29m 40s
test (7.3, MOODLE_311_STABLE, pgsql, SBCL)	18m 14s
test (7.2, MOODLE_39_STABLE, pgsql, SBCL)	28m 8s
	3h 40m 57s

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