Passing formulas to OpenSCAD modules/functions

This page is mainly for reference. Most users will want this simpler solution which parses strings containing OpenSCAD formulas: http://www.thingiverse.com/thing:2295309

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One of the features that OpenSCAD is lacking is the ability to pass functions to modules or to functions. This is a quick-and-dirty proof-of-concept solution to the not uncommon problem of wanting to pass a mathematical formula to a module or function: an eval() function that evaluates expressions in a simple LISP-inspired language that supports all of the OpenSCAD mathematical operators. The evaluation is moderately fast: about 10,000 evaluations of the sample formula below per second on my laptop. The downside is that it's pretty inconvenient to write expressions in this language.

Call the function with: `eval(expression, variables)`. Here, `variables` is a table of variable values, where each line of the table has the name of the variable followed by its value, e.g., [["x", 12.3], ["y", 7]].

An `exp