+This statement is a GNU extension.
+.El
+.Pp
+Operation keywords have the following semantics:
+.Bl -tag -width Ds
+.It Cm above
+See
+.Cm pile .
+.It Cm bar
+Draw a line over the preceding box.
+.It Cm bold
+Set the following box using bold font.
+.It Cm ccol
+Like
+.Cm cpile ,
+but for use in
+.Cm matrix .
+.It Cm cpile
+Like
+.Cm pile ,
+but with slightly increased vertical spacing.
+.It Cm dot
+Set a single dot over the preceding box.
+.It Cm dotdot
+Set two dots (dieresis) over the preceding box.
+.It Cm dyad
+Set a dyad symbol (left-right arrow) over the preceding box.
+.It Cm fat
+A synonym for
+.Cm bold .
+.It Cm font
+Set the second argument using the font specified by the first argument;
+currently not recognized by the
+.Xr mandoc 1
+.Nm
+parser.
+.It Cm from
+Set the following box below the preceding box,
+using a slightly smaller font.
+Used for sums, integrals, limits, and the like.
+.It Cm hat
+Set a hat (circumflex) over the preceding box.
+.It Cm italic
+Set the following box using italic font.
+.It Cm lcol
+Like
+.Cm lpile ,
+but for use in
+.Cm matrix .
+.It Cm left
+Set the first argument as a big left delimiter before the second argument.
+As an optional third argument,
+.Cm right
+can follow.
+In that case, the fourth argument is set as a big right delimiter after
+the second argument.
+.It Cm lpile
+Like
+.Cm cpile ,
+but subequations are left-justified.
+.It Cm matrix
+Followed by a list of columns enclosed in braces.
+All columns need to have the same number of subequations.
+The columns are set as a matrix.
+The difference compared to multiple subsequent
+.Cm pile
+operators is that in a
+.Cm matrix ,
+corresponding subequations in all columns line up horizontally,
+while each
+.Cm pile
+does vertical spacing independently.
+.It Cm over
+Set a fraction.
+The preceding box is the numerator, the following box is the denominator.
+.It Cm pile
+Followed by a list of subequations enclosed in braces,
+the subequations being separated by
+.Cm above
+keywords.
+Sets the subequations one above the other, each of them centered.
+Typically used to represent vectors in coordinate representation.
+.It Cm rcol
+Like
+.Cm rpile ,
+but for use in
+.Cm matrix .
+.It Cm right
+See
+.Cm left ;
+.Cm right
+cannot be used without
+.Cm left .
+To set a big right delimiter without a big left delimiter, the following
+construction can be used:
+.Pp
+.D1 Cm left No \(dq\(dq Ar box Cm right Ar delimiter
+.It Cm roman
+Set the following box using the default font.
+.It Cm rpile
+Like
+.Cm cpile ,
+but subequations are right-justified.
+.It Cm size
+Set the second argument with the font size specified by the first
+argument; currently ignored by
+.Xr mandoc 1 .
+By prepending a plus or minus sign to the first argument,
+the font size can be selected relative to the current size.
+.It Cm sqrt
+Set the square root of the following box.
+.It Cm sub
+Set the following box as a subscript to the preceding box.
+.It Cm sup
+Set the following box as a superscript to the preceding box.
+As a special case, if a
+.Cm sup
+clause immediately follows a
+.Cm sub
+clause as in
+.Pp
+.D1 Ar mainbox Cm sub Ar subbox Cm sup Ar supbox
+.Pp
+both are set with respect to the same
+.Ar mainbox ,
+that is,
+.Ar supbox
+is set above
+.Ar subbox .
+.It Cm tilde
+Set a tilde over the preceding box.
+.It Cm to
+Set the following box above the preceding box,
+using a slightly smaller font.
+Used for sums and integrals and the like.
+As a special case, if a
+.Cm to
+clause immediately follows a
+.Cm from
+clause as in
+.Pp
+.D1 Ar mainbox Cm from Ar frombox Cm to Ar tobox
+.Pp
+both are set below and above the same
+.Ar mainbox .
+.It Cm under
+Underline the preceding box.
+.It Cm vec
+Set a vector symbol (right arrow) over the preceding box.
+.El
+.Pp
+The binary operations
+.Cm from ,
+.Cm to ,
+.Cm sub ,
+and
+.Cm sup
+group to the right, that is,
+.Pp
+.D1 Ar mainbox Cm sup Ar supbox Cm sub Ar subbox
+.Pp
+is the same as
+.Pp
+.D1 Ar mainbox Cm sup Brq Ar supbox Cm sub Ar subbox
+.Pp
+and different from
+.Pp
+.D1 Bro Ar mainbox Cm sup Ar supbox Brc Cm sub Ar subbox .
+.Pp
+By contrast,
+.Cm over
+groups to the left.
+.Pp
+In the following list, earlier operations bind more tightly than
+later operations:
+.Pp
+.Bl -enum -compact
+.It
+.Cm dyad ,
+.Cm vec ,
+.Cm under ,
+.Cm bar ,
+.Cm tilde ,
+.Cm hat ,
+.Cm dot ,
+.Cm dotdot
+.It
+.Cm fat ,
+.Cm roman ,
+.Cm italic ,
+.Cm bold ,
+.Cm size
+.It
+.Cm sub ,
+.Cm sup
+.It
+.Cm sqrt
+.It
+.Cm over
+.It
+.Cm from ,
+.Cm to