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Expression evaluation instruction

Expression evaluation using an extended version of Dijkstra's "shunting
yard" algorithm with JavaScript semantics.  Dijkstra's algorithm was
chosen as it is simple, sufficient, and can be implemented compactly,
minimizing the size of the code and making it easier to verify correct.

Features:

 - Number, Boolean, Null, and String types
 - String parsing of backslash char, hex, and unicode escapes
   - "\n", "\x20", "\u2018", "\u0001f600" are all valid strings
 - Decimal and hex numbers
 - Assignment statements
 - Variable references
 - Operator precedence
 - Common math, binary and logical operators
 - Strict equality / inequality only
 - Nesting of sub-expressions with parenthesis
 - Single pass evaluation of multiple semicolon-delimited expressions
 - String equality and concatenation
 - Function calls with a variable number of arguments
   - example: max(1, 2, 3, ..., N) is accepted
 - Type conversion functions
 - Common constants (pi, e)

The algorithm assumes the expression is well-formed and performs minimal
validation.  A malformed expression should either terminate evaluation
early or produce no value.

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