Editorial: clarify that named alias (Let) are reference-like

Closes #752.

spec.html

@@ -706,6 +706,7 @@ <h1>Algorithm Conventions</h1>
     <p>A step or substep may be written as an &ldquo;if&rdquo; predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word &ldquo;else&rdquo;, it is a predicate that is the negation of the preceding &ldquo;if&rdquo; predicate step at the same level.</p>
     <p>A step may specify the iterative application of its substeps.</p>
     <p>A step that begins with &ldquo;<dfn id="assert">Assert</dfn>:&rdquo; asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.</p>
+    <p>Algorithm steps may declare named aliases for any value using the form &ldquo;Let _x_ be _someValue_&rdquo;. These aliases are reference-like in that both _x_ and _someValue_ refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behavior should explicitly make a copy of the right-hand side: &ldquo;Let _x_ be a copy of _someValue_&rdquo; creates a shallow copy of _someValue_.</p>
     <p>Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which unless otherwise noted do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is *+0* or *-0* then the corresponding mathematical value is simply 0.</p>
     <p>The mathematical function <emu-eqn aoid="abs">abs(_x_)</emu-eqn> produces the absolute value of _x_, which is <emu-eqn>-_x_</emu-eqn> if _x_ is negative (less than zero) and otherwise is _x_ itself.</p>
     <p>The mathematical function <emu-eqn aoid="min">min(_x1_, _x2_, ..., _xN_)</emu-eqn> produces the mathematically smallest of <emu-eqn>_x1_</emu-eqn> through <emu-eqn>_xN_</emu-eqn>. The mathematical function <emu-eqn aoid="max">max(_x1_, _x2_, ..., _xN_)</emu-eqn> produces the mathematically largest of <emu-eqn>_x1_</emu-eqn> through <emu-eqn>_xN_</emu-eqn>. The domain and range of these mathematical functions include *+&infin;* and *-&infin;*.</p>