Condition BNF

<condition>           ::= <expression> | <expression-list>
<expression>          ::= { <field> : { <comparison-operator> : <value> } }
<expression-list>     ::= { <boolean-operator> : [ <condition> , <condition> {, <condition>} ] }
<pair-comparison>     ::= { <field> : { <comparison-operator> : <value> } }
<array-comparison>    ::= { <field> : { $in : [ <value> {, <value>} ] } }
<comparison-operator> ::= $eq | $neq | $lt | $lte | $gt | $gte
<boolean-operator>    ::= $and | $or
<field>               ::= <text>
<value>               ::= <text> | <number>

/****
 * Some examples.
****/
// `name` = 'Sam'
{name: {$eq: 'Sam'}}

// `age` > 21
{age: {$gt: 21}}

// `showSize` IN (10, 10.5, 11)
{shoeSize: {$in: [10, 10.5, 11]}}

// (`name` = 'Sam' AND `age` > 21 AND showSize IN (10, 10.5, 11))
{$and: [{name: {$eq: 'Sam'}}, {age: {$gt: 21}}, {shoeSize: {$in: [10, 10.5, 11]}}]}

// (`name` = 'Sam' AND (`age` > 21 OR showSize IN (10, 10.5, 11)))
{$and: [{name: {$eq: 'Sam'}}, {$or: [{age: {$gt: 21}}, {shoeSize: {$in: [10, 10.5, 11]}}]}]}