[concepts] Rephrase ocurrences of "must"

Author: CaseyCarter

source/concepts.tex

@@ -30,7 +30,7 @@
 is the expression's result and all operands modified by the expression.
 
 \pnum
-Not all input values must be valid for a given expression; e.g., for integers
+Not all input values need be valid for a given expression; e.g., for integers
 \tcode{a} and \tcode{b}, the expression \tcode{a / b} is not well-defined when
 \tcode{b} is \tcode{0}. This does not preclude the expression \tcode{a / b}
 being equality-preserving. The \term{domain} of an expression is the set of
@@ -39,8 +39,8 @@
 \pnum
 Expressions required by this document to be equality-preserving are further
 required to be stable: two evaluations of such an expression with the same input
-objects must have equal outputs absent any explicit intervening modification of
-those input objects.
+objects are required to have equal outputs absent any explicit intervening
+modification of those input objects.
 \begin{note}
 This requirement allows generic code to reason about the current values of
 objects based on knowledge of the prior values as observed via
@@ -64,18 +64,18 @@
 This document uses a notational convention to specify which expressions declared
 in a \grammarterm{requires-expression} modify which inputs: except where
 otherwise specified, an expression operand that is a non-constant lvalue or
-rvalue may be modified. Operands that are constant lvalues or rvalues must not
-be modified.
+rvalue may be modified. Operands that are constant lvalues or rvalues are
+required to not be modified.
 
 \pnum
 Where a \grammarterm{requires-expression} declares an expression that is
 non-modifying for some constant lvalue operand, additional variations of that
 expression that accept a non-constant lvalue or (possibly constant) rvalue for
 the given operand are also required except where such an expression variation is
 explicitly required with differing semantics. These
-\term{implicit expression variations} must meet the semantic requirements of the
-declared expression. The extent to which an implementation validates the syntax
-of the variations is unspecified.
+\term{implicit expression variations} are required to meet the semantic
+requirements of the declared expression. The extent to which an implementation
+validates the syntax of the variations is unspecified.
 
 \begin{example}
 \begin{codeblock}
@@ -333,7 +333,8 @@
 \pnum
 The \libconcept{ConvertibleTo} concept requires an expression of a particular
 type and value category to be both implicitly and explicitly convertible to some
-other type. The implicit and explicit conversions must produce equal results.
+other type. The implicit and explicit conversions are required to produce equal
+results.
 
 \indexlibrary{\idxcode{ConvertibleTo}}%
 \begin{itemdecl}

source/lib-intro.tex

@@ -967,7 +967,7 @@
 the same name in namespace \tcode{std}, customization point objects specify that
 lookup for these expressions is performed in a context that includes deleted
 overloads matching the signatures of overloads defined in namespace \tcode{std}.
-When the deleted overloads are viable, user-defined overloads must be more
+When the deleted overloads are viable, user-defined overloads need be more
 specialized\iref{temp.func.order} or more constrained\iref{temp.constr.order} to
 be used by a customization point object.
 \end{note}