 arithmetic expression.  Evaluation is done in
    fixed-width integers with no check for overflow, though division by 0
    is trapped and flagged as an error.  The following list of operators is
    grouped into levels of equal-precedence operators.  The levels are listed
    in order of decreasing precedence.
    
    	id++, id--	variable post-increment, post-decrement
    	++id, --id	variable pre-increment, pre-decrement
    	-, +		unary minus, plus
    	!, ~		logical and bitwise negation
    	**		exponentiation
    	*, /, %		multiplication, division, remainder
    	+, -		addition, subtraction
    	<<, >>		left and right bitwise shifts
    	<=, >=, <, >	comparison
    	==, !=		equality, inequality
    	&		bitwise AND
    	^		bitwise XOR
    	|		bitwise OR
    	&&		logical AND
    	||		logical OR
    	expr ? expr : expr
    			conditional operator
    	=, *=, /=, %=,
    	+=, -=, <<=, >>=,
    	&=, ^=, |=	assignment
    
    Shell variables are allowed as operands.  The name of the variable
    is replaced by its value (coerced to a fixed-width integer) within
    an expression.  The variable need not have its integer attribute
    turned on to be used in an expression.
    
    Operators are evaluated in order of precedence.  Sub-expressions in
    parentheses are evaluated first and may override the precedence
    rules above.
    
    Exit Status:
    If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise.