First attempt to fix issue #419.

doc/en/Authoring/Inputs.md

doc/en/Developer/Development_track.md

lang/en/qtype_stack.php

renderer.php

samplequestions/input-sample-questions.xml

stack/cas/casstring.class.php

stack/input/algebraic/algebraic.class.php

stack/input/inputbase.class.php

stack/input/numerical/numerical.class.php

tests/input_algebraic_test.php